Fix to run on Heroku (#236)

* Specify a Node.js version

Because Heroku is not recommended a semver range (4.x) in production.
https://devcenter.heroku.com/articles/nodejs-support#specifying-a-node-js-version

* Move Babel and Gulp modules from devDependencies into dependencies

To run `gulp` command in Heroku production.

* Replace prepublish with postinstall

To run gulp command in Heroku production.
https://devcenter.heroku.com/articles/node-best-practices#hook-things-up

package.json

@@ -15,12 +15,19 @@
   },
   "dependencies": {
     "args": "1.3.0",
+    "babel-core": "6.3.26",
+    "babel-eslint": "6.0.4",
     "babel-polyfill": "6.3.14",
+    "babel-preset-es2015": "6.3.13",
+    "babel-register": "6.9.0",
     "body-parser": "1.10.2",
     "cors": "2.7.1",
     "debug": "2.1.1",
     "email-regex": "1.0.0",
     "express": "4.11.0",
+    "gulp-babel": "6.1.1",
+    "gulp-rimraf": "0.2.0",
+    "gulp": "3.9.0",
     "hostenv": "1.0.1",
     "opentype.js": "0.4.4",
     "socket.io": "1.4.8",
@@ -29,21 +36,14 @@
   },
   "license": "MIT",
   "devDependencies": {
-    "babel-core": "6.3.26",
-    "babel-preset-es2015": "6.3.13",
-    "babel-eslint": "6.0.4",
-    "babel-register": "6.9.0",
-    "gulp": "3.9.0",
-    "gulp-babel": "6.1.1",
-    "gulp-rimraf": "0.2.0",
     "eslint": "2.12.0",
     "eslint-config-default": "0.2.0",
     "mocha": "2.2.4",
     "nock": "2.17.0",
     "supertest": "0.15.0"
   },
   "engines": {
-    "node": ">=0.12.0"
+    "node": "4.5.0"
   },
   "bin": {
     "slackin": "./bin/slackin"
@@ -57,6 +57,6 @@
   },
   "scripts": {
     "test": "mocha && eslint lib/**",
-    "prepublish": "gulp"
+    "postinstall": "gulp"
   }
 }