Remove ruby-2.2.2 on xcode6.2 from Travis build

This was building an image for darwin14 instead of 13 which makes it
kind of useless. Remove to improve build speed.

Author: ignisf

.travis.yml

@@ -12,9 +12,6 @@ matrix:
     - rvm: ruby-2.2.5
       os: osx
       osx_image: xcode6.4
-    - rvm: ruby-2.2.2
-      os: osx
-      osx_image: beta-xcode6.2
     - rvm: system
       os: osx
       osx_image: xcode7.3