Remove postinstall step

Use a `prepare` hook instead of a `postinstall` hook to generate
the `highlight_alias.json` file. This means that the file is generated
during the publication of package to npm or when manually running
`npm install` in the project directory.
The previous behavior was to generate this step on the consumer
side: this was the cause of numerous hard-to-reproduce issues due
to the variety of environments.

Closes #20
Closes #23
Closes #24

.npmignore

@@ -7,4 +7,3 @@ coverage/
 gulpfile.js
 .idea/
 appveyor.yml
-highlight_alias.json

.travis.yml

@@ -11,7 +11,10 @@ node_js:
   - "6"
   - "node"
 
+# For older versions of npm (ie. the one bundled with Node 6), we need to manually run `prepare`
+# to build the project, hence the first line. You can still install and consume the package without any issue.
 script:
+  - [[ "$TRAVIS_NODE_VERSION" == "6" ]] && npm run prepare
   - npm run eslint
   - npm run jscs
   - npm run test-cov

CHANGELOG.md

@@ -0,0 +1,3 @@
+# Next
+
+- **[Fix]** Remove postinstall step, fixes installations issues for some systems. [#24](https://github.com/hexojs/hexo-util/issues/24)