[Security] Bump sshpk from 1.13.1 to 1.15.1 (#63)

Bumps [sshpk](https://github.com/joyent/node-sshpk) from 1.13.1 to 1.15.1. **This update includes security fixes.**
<details>
<summary>Vulnerabilities fixed</summary>

*Sourced from [The Sonatype OSS Index](https://ossindex.sonatype.org/vuln/fc393f9f-282f-4bc9-953b-d7e4b48352e9).*

> **CWE-400: Uncontrolled Resource Consumption ('Resource Exhaustion')**
> The software does not properly restrict the size or amount of resources that are requested or influenced by an actor, which can be used to consume more resources than intended.
> 
> Affected versions: <1.14.1

*Sourced from The GitHub Vulnerability Alert Database.*

> **CVE-2018-3737**
> See https://nvd.nist.gov/vuln/detail/CVE-2018-3737.
> 
> Affected versions: < 1.13.2

*Sourced from [The Node Security Working Group](https://github.com/nodejs/security-wg/blob/master/vuln/npm/401.json).*

> **Denial of Service**
> `sshpk` is vulnerable to ReDoS when parsing crafted invalid public keys
> 
> Affected versions: <=1.13.1

</details>
<details>
<summary>Release notes</summary>

*Sourced from [sshpk's releases](https://github.com/joyent/node-sshpk/releases).*

> ## v1.14.1
>  * Remove all remaining usage of jodid25519 (abandoned dep)
>  * Add support for DNSSEC key format
>  * Add support for Ed25519 keys in PEM format (according to draft-curdle-pkix)
>  * Fixes for X.509 encoding issues (asn.1 NULLs in RSA certs, cert string type mangling)
>  * Performance issues parsing long SSH public keys
</details>
<details>
<summary>Commits</summary>

- [`2ab4f2a`](TritonDataCenter/node-sshpk@2ab4f2a) TritonDataCenter/node-sshpk#56 md5 fingerprints not quite right
- [`026ef47`](TritonDataCenter/node-sshpk@026ef47) TritonDataCenter/node-sshpk#53 stop using optional deps to fix webpack
- [`53e23fe`](TritonDataCenter/node-sshpk@53e23fe) TritonDataCenter/node-sshpk#50 Support PKCS#5 AES-256-CBC encrypted private keys
- [`6b68d49`](TritonDataCenter/node-sshpk@6b68d49) TritonDataCenter/node-sshpk#54 want API for accessing x509 extensions
- [`1088992`](TritonDataCenter/node-sshpk@1088992) TritonDataCenter/node-sshpk#52 Buffer no longer performs length check for hex strings i...
- [`6ec6f9d`](TritonDataCenter/node-sshpk@6ec6f9d) TritonDataCenter/node-sshpk#38 want support for more obscure DN OIDs
- [`1cc4c99`](TritonDataCenter/node-sshpk@1cc4c99) TritonDataCenter/node-sshpk#51 package.json repository does not point to Joyent
- [`175758a`](TritonDataCenter/node-sshpk@175758a) TritonDataCenter/node-sshpk#46 Use Buffer.(from|alloc) instead of deprecated Buffer API
- [`6edb37c`](TritonDataCenter/node-sshpk@6edb37c) Release 1.14.0
- [`46065d3`](TritonDataCenter/node-sshpk@46065d3) TritonDataCenter/node-sshpk#44 Performance issues parsing long SSH public keys
- Additional commits viewable in [compare view](TritonDataCenter/node-sshpk@v1.13.1...v1.15.1)
</details>
<br />

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<details>
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</details>

yarn.lock

@@ -2731,7 +2731,7 @@ safe-regex@^1.1.0:
   dependencies:
     ret "~0.1.10"
 
-"safer-buffer@>= 2.1.2 < 3":
+"safer-buffer@>= 2.1.2 < 3", safer-buffer@^2.0.2:
   version "2.1.2"
   resolved "https://registry.yarnpkg.com/safer-buffer/-/safer-buffer-2.1.2.tgz#44fa161b0187b9549dd84bb91802f9bd8385cd6a"
 
@@ -2913,17 +2913,17 @@ sprintf-js@~1.0.2:
   resolved "https://registry.yarnpkg.com/sprintf-js/-/sprintf-js-1.0.3.tgz#04e6926f662895354f3dd015203633b857297e2c"
 
 sshpk@^1.7.0:
-  version "1.13.1"
-  resolved "https://registry.yarnpkg.com/sshpk/-/sshpk-1.13.1.tgz#512df6da6287144316dc4c18fe1cf1d940739be3"
+  version "1.15.1"
+  resolved "https://registry.yarnpkg.com/sshpk/-/sshpk-1.15.1.tgz#b79a089a732e346c6e0714830f36285cd38191a2"
   dependencies:
     asn1 "~0.2.3"
     assert-plus "^1.0.0"
-    dashdash "^1.12.0"
-    getpass "^0.1.1"
-  optionalDependencies:
     bcrypt-pbkdf "^1.0.0"
+    dashdash "^1.12.0"
     ecc-jsbn "~0.1.1"
+    getpass "^0.1.1"
     jsbn "~0.1.0"
+    safer-buffer "^2.0.2"
     tweetnacl "~0.14.0"
 
 stack-utils@^1.0.1: