Configure Travis CI to cache phantomjs binaries

This should speed up our CI builds and also save a lot of bandwidth
for people providing phantomjs-prebuilt module.

See also
https://www.npmjs.com/package/phantomjs-prebuilt#continuous-integration

.travis.yml

@@ -6,3 +6,15 @@ node_js:
   - "4"
   - "6"
 
+# see https://www.npmjs.com/package/phantomjs-prebuilt#continuous-integration
+cache:
+  directories:
+    - 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"