Makes npm install run gulp build --prod

To avoid issues such as joemccann#514 and to allow Travis CI
to run unit test.

package.json

@@ -19,7 +19,7 @@
   ],
   "scripts": {
     "start": "node app.js",
-    "predeploy": "gulp build --prod"
+    "postinstall": "gulp build --prod"
   },
   "keywords": [
     "dillinger",
@@ -73,6 +73,21 @@
     "markdown-it-sup": "^1.0.0",
     "markdown-it-toc": "^1.1.0",
     "method-override": "^2.3.5",
+    "gulp": "^3.9.0",
+    "gulp-autoprefixer": "^3.0.2",
+    "gulp-combine-media-queries": "^0.2.0",
+    "gulp-csso": "^0.2.9",
+    "gulp-filename-media-query": "^1.2.1",
+    "gulp-group-css-media-queries": "^1.1.0",
+    "gulp-if": "2.0.0",
+    "gulp-notify": "^2.2.0",
+    "gulp-open": "^0.3.0",
+    "gulp-rev-mtime": "^0.0.4",
+    "gulp-rimraf": "0.2.0",
+    "gulp-sass": "^2.0.4",
+    "gulp-size": "^1.1.0",
+    "gulp-uncss": "^0.5.1",
+    "gulp-util": "^3.0.6",
     "morgan": "^1.6.1",
     "phantom": "^2.0.4",
     "rc": "0.3.0",
@@ -92,21 +107,6 @@
     "dom-serializer": "^0.1.0",
     "exports-loader": "^0.6.2",
     "glob": "^5.0.5",
-    "gulp": "^3.9.0",
-    "gulp-autoprefixer": "^3.0.2",
-    "gulp-combine-media-queries": "^0.2.0",
-    "gulp-csso": "^0.2.9",
-    "gulp-filename-media-query": "^1.2.1",
-    "gulp-group-css-media-queries": "^1.1.0",
-    "gulp-if": "2.0.0",
-    "gulp-notify": "^2.2.0",
-    "gulp-open": "^0.3.0",
-    "gulp-rev-mtime": "^0.0.4",
-    "gulp-rimraf": "0.2.0",
-    "gulp-sass": "^2.0.4",
-    "gulp-size": "^1.1.0",
-    "gulp-uncss": "^0.5.1",
-    "gulp-util": "^3.0.6",
     "htmlparser2": "^3.8.3",
     "jshint-stylish": "^1.0.0",
     "json-loader": "^0.5.1",