Use a prepare script for transpiling instead of prepublish (#323)

From Travis logs:
As of npm@5, `prepublish` scripts will run only for `npm publish`.
(In npm@4 and previous versions, it also runs for `npm install`.)
See the deprecation note in `npm help scripts` for more information.

This was also causing discord-irc to fail to transpile when installed
using npm from source.

This modifies the script to run on the `prepare` hook, so it should
transpile when installed directly from source too.

package.json

@@ -25,7 +25,7 @@
   "scripts": {
     "start": "node dist/index.js",
     "build": "babel lib --out-dir dist",
-    "prepublish": "npm run build",
+    "prepare": "npm run build",
     "lint": "eslint . --ignore-path .gitignore",
     "coverage": "nyc --require babel-core/register _mocha -- test/*.test.js",
     "report": "nyc report --reporter=text-lcov | coveralls",