chore(build): cache bower dir & rm global node pkg

- Caches the installed Bower dependencies for faster builds.
- Removes global Node.JS modules gulp and bower from travis conf & make
	them devDependencies for more reproducable and faster builds.

.travis.yml

@@ -19,15 +19,14 @@
 
 # Explicitly set the environment to be container-based. This makes the builds faster.
 sudo: false
-# Cache downloaded Node.JS modules in the node_modules directory for faster builds.
+# Cache downloaded Node.JS modules & Bower frontend dependencies for faster builds.
 cache:
   directories:
     - node_modules
+    - bower_components
 # Use Node.js as primary language because Gulp is the build system used in the project.
 language: node_js
 before_script:
-  # Install dependencies required to setup the build.
-  - npm install -g bower
-  - npm install -g gulp
-  - bower install
-script: gulp build
+  # Install frontend dependencies to be able to build the project.
+  - ./node_modules/.bin/bower install
+script: ./node_modules/.bin/gulp build

package.json

@@ -8,9 +8,10 @@
     "babel-core": "~5.8.25",
     "babel-loader": "~5.3.2",
     "babelify": "~6.3.0",
-    "browserify": "~11.2.0",
+    "bower": "~1.6.5",
     "browser-sync": "~2.9.2",
     "browser-sync-spa": "~1.0.2",
+    "browserify": "~11.2.0",
     "del": "~2.0.1",
     "eslint-loader": "~1.0.0",
     "eslint-plugin-angular": "~0.11.0",