Vendor PhantomJS 2.1.1 in our repository

As of today Travis CI supports PhantomJS 1.9.
Previously, the CI script (in `.travis.yml`) tried to fetch it before
each build.

But this is inefficient and the official channel of distribution for
PhantomJS ofter drops the connection and the build fails miseably.

When Travis CI will support PhantomJS 2.1 we can remove it and review
the CI setup again.

.travis.yml

@@ -3,10 +3,8 @@ sudo: false
 cache: bundler
 install: true
 before_install:
-  - wget https://bitbucket.org/ariya/phantomjs/downloads/phantomjs-2.1.1-linux-x86_64.tar.bz2
-  - tar -xjf phantomjs-2.1.1-linux-x86_64.tar.bz2
-  - mv phantomjs-2.1.1-linux-x86_64 phantomjs
-  - export PATH=$PWD/phantomjs/bin:$PATH
+  - mkdir $PWD/vendor/phantomjs && tar -xjf $PWD/vendor/phantomjs-2.1.1-linux-x86_64.tar.bz2 -C $PWD/vendor/phantomjs --strip-components=1
+  - export PATH=$PWD/vendor/phantomjs/bin:$PATH
   - phantomjs --version
 env:
   global: