Remove xcode 6.4 images from Travis

There seems to be a compilation faulure due to a missing gcc_s.10.5 library. The
only suggested fixes revolve around updating xcode, which is not an option for
now.

Author: ignisf

.travis.yml

@@ -12,18 +12,12 @@ matrix:
     - rvm: 2.4.0
       os: osx
       osx_image: xcode7.3
-    - rvm: 2.4.0
-      os: osx
-      osx_image: xcode6.4
     - rvm: system
       os: osx
       osx_image: xcode8.2
     - rvm: system
       os: osx
       osx_image: xcode7.3
-    - rvm: system
-      os: osx
-      osx_image: xcode6.4
     - rvm: ruby-2.3.2-clang
       env: CXX=clang++
     - rvm: 1.9.3