Remove outdated phantomjs install instruction from CONTRIBUTING.md

Author: Shreyaaaash

CONTRIBUTING.md

@@ -58,7 +58,9 @@ CodeTriage requires Redis for background processing.
 If you're on OS X, Homebrew is the simplest way to install Redis:
 
 ```shell
-$ brew install redis memcached && brew cask install phantomjs
+
+$ brew install redis memcached
+
 $ redis-server
 ```