Update travis

.travis.yml

 sudo: false
 language: node_js
 node_js:
-  - "5"
-script: make dist
+  - 5
+cache:
+  bundler: true
+  directories:
+    - node_modules # NPM packages
+    - travis_phantomjs
+before_install:
+  # Upgrade PhantomJS to v2.1.1.
+  - "export PHANTOMJS_VERSION=2.1.1"
+  - "export PATH=$PWD/travis_phantomjs/phantomjs-$PHANTOMJS_VERSION-linux-x86_64/bin:$PATH"
+  - "if [ $(phantomjs --version) != $PHANTOMJS_VERSION ]; then rm -rf $PWD/travis_phantomjs; mkdir -p $PWD/travis_phantomjs; fi"
+  - "if [ $(phantomjs --version) != $PHANTOMJS_VERSION ]; then wget https://github.com/Medium/phantomjs/releases/download/v$PHANTOMJS_VERSION/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2 -O $PWD/travis_phantomjs/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2; fi"
+  - "if [ $(phantomjs --version) != $PHANTOMJS_VERSION ]; then tar -xvf $PWD/travis_phantomjs/phantomjs-$PHANTOMJS_VERSION-linux-x86_64.tar.bz2 -C $PWD/travis_phantomjs; fi"
+  - "phantomjs --version"
+script:
+  - "make dist"
+  - "npm t"