Refactor build routine and switch to airbnb style (#1703)

- Relocate webpack stuff to assets/build
- Switch all js to airbnb code style
- Update webpack to v2.0 (beta)
- Update all other dependencies
- Assets manifest uses relative path as key [1]
- Code should be compatible with Node 4.5.0
- All dependencies have been updated

[1]: Previously the assets manifest would use the basename of a file path as
its key when looking up a cachebusted file. This was to be consistent with
Sage 8. This change makes it so that the relative path is used instead.

To clarify, review the following example.

Before:
```
{
  "main.js": "scripts/main_5f4bfc9a9f82291c6dea.js",
  "main.css": "styles/main_5f4bfc9a9f82291c6dea.css",
  "customizer.js": "scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

After:
```
{
  "scripts/main.js":"scripts/main_5f4bfc9a9f82291c6dea.js",
  "styles/main.css":"styles/main_5f4bfc9a9f82291c6dea.css",
  "scripts/customizer.js":"scripts/customizer_5f4bfc9a9f82291c6dea.js"
}
```

.travis.yml

@@ -7,8 +7,8 @@ php:
   - nightly
 
 env:
-  - TRAVIS_NODE_VERSION="4.2" CXX="g++-4.8"
-  - TRAVIS_NODE_VERSION="5.3" CXX="g++-4.8"
+  - TRAVIS_NODE_VERSION="4" CXX="g++-4.8"
+  - TRAVIS_NODE_VERSION="6" CXX="g++-4.8"
 
 matrix:
   fast_finish: true
@@ -33,7 +33,7 @@ before_install:
   - composer self-update
 
 install:
-  - source ~/.nvm/nvm.sh && nvm install $TRAVIS_NODE_VERSION
+  - source ~/.nvm/nvm.sh && nvm install $TRAVIS_NODE_VERSION && nvm use $TRAVIS_NODE_VERSION 
   - travis_retry npm install -g npm@latest
   - travis_retry npm install -g eslint
   - node -v && npm -v
@@ -43,5 +43,8 @@ install:
   - composer install -o --prefer-dist --no-interaction
 
 script:
+  - npm run test
   - npm run build
+  - npm run clean
+  - npm run "build:production"
   - composer test