hub: use system rake to avoid RVM's Bundler integration

Fixes defunkt/hub#205

Closes Homebrew/homebrew#13238.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>

Formula/hub.rb

@@ -7,11 +7,22 @@ class Hub < Formula
   md5 'd3ff9bcf489466a408020867b77f1d4d'
 
   def install
-    system "rake", "install", "prefix=#{prefix}"
+    system rake_bin, "install", "prefix=#{prefix}"
     (prefix+'etc/bash_completion.d').install 'etc/hub.bash_completion.sh'
     (share+'zsh/site-functions').install 'etc/hub.zsh_completion' => '_hub'
   end
 
+  def rake_bin
+    require 'rbconfig'
+    ruby_rake = File.join RbConfig::CONFIG['bindir'], 'rake'
+
+    if File.exist? ruby_rake
+      ruby_rake
+    else
+      '/usr/bin/rake'
+    end
+  end
+
   def caveats; <<-EOS.undent
     Bash completion has been installed to:
       #{etc}/bash_completion.d