Update dependencies and health files and fix linter problems

[SethFalco]

.github/ISSUE_TEMPLATE.md

• Removes Slack as this doesn't appear to be running. The link was also removed from the base metalsmith repository. (metalsmith/metalsmith@0bea2ac)

README.md

• Removes Greenkeeper as this is no longer active in favor of Snyk.

lib/index.js

• Fixed linter problems introduced after updating dependencies. (prefer-object-spread)

package.json

• Update all dependencies to their latest versions except multimatch which I only updated to 5.0.0. (Latest is 6.0.0)
  • Reduces vulnerabilities from 42 moderate severity vulnerabilities to 5 moderate severity vulnerabilities

---

Linting and tests all pass after these changes.

[webketje]

Thx @SethFalco,

I have 1 main poinrt before this can be merged:

• For the next release I'd like to keep Node >= 8 support. You can check dependency compatibility by running npm info <pkg>@* engines.node. Unfortunately the latest Node 8 compatible version for multimatch is 4.0.0, so that needs to be re-downgraded. For devDependecies the ones used in CI npm test need to be compatible with the environments they are used in. That means [email protected] and possibly others. Others can be upgraded to latest.

Stackoverflow link can be https.
I don't really agree with the ESlint rule but I don't mind following it.

[SethFalco]

Sure, I've added the following to the package.json also just to kind of document that requirement.
I'm not sure if npm uses it, but I know yarn would automatically check against the engines when installing dependencies.

"engines": {
  "node": ">=8"
},

• I've downgrading dependencies to meet the engine.node field of dependencies.
• Replaced all http links with https.
• Removed a reference to the Slack channel from the README which I just noticed.

Just noting it since it's changed from the original PR summary:
Reduces vulnerabilities from 42 moderate severity vulnerabilities to 19 vulnerabilities (16 moderate, 3 critical)

[SethFalco]

The build fails because of the test task running with Node 6. If we're only supporting Node >= 8, I assume we can remove that and rerun this?

[webketje]

@SethFalco Correct, I'm in the process of upgrading the plugin repo's to a common setup and that will include the travis.yml currently already available at https://github.com/metalsmith/excerpts/blob/master/.travis.yml