.travis.yml: only build on latest OS X. (Homebrew#74)

We have Jenkins to test the other versions.

.travis.yml

@@ -1,19 +1,9 @@
 # Force OS X.
 language: objective-c
-matrix:
-  include:
-    - env: OSX=10.11
-      os: osx
-      osx_image: xcode7.2
-      rvm: system
-    - env: OSX=10.10
-      os: osx
-      osx_image: xcode7.1
-      rvm: system
-    - env: OSX=10.9 HOMEBREW_RUBY=1.8.7
-      os: osx
-      osx_image: beta-xcode6.2
-      rvm: system
+os: osx
+env: OSX=10.11
+osx_image: xcode7.3
+rvm: system
 
 before_install:
   - export TRAVIS_COMMIT=$(git rev-parse --verify -q HEAD)