Use PhantomJS 2.1.1 in Travis

**Why**: To use the latest and greatest, and for a better experience, as noted by the Poltergeist gem.

.travis.yml

@@ -2,7 +2,17 @@ language: ruby
 bundler_args: "--without development:production"
 cache: bundler
 sudo: false
-before_install: gem update --system
+before_install:
+- gem update --system
+# Install PhantomJS 2.1.1 manually
+- "export PHANTOMJS_VERSION=2.1.1"
+- "phantomjs --version"
+- "export PATH=$PWD/travis_phantomjs/phantomjs-$PHANTOMJS_VERSION-linux-x86_64/bin:$PATH"
+- "phantomjs --version"
+- "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: bundle exec rspec
 rvm:
 - 2.3.3