travis.yml: cache Bundler output.

Travis needs some coaxing to find where we put it. This should improve
build reliability due to network issues.

.travis.yml

 language: ruby
+cache:
+  directories:
+    - $HOME/.gem/ruby
+    - Library/Homebrew/vendor/bundle
 
 matrix:
   include:
     - os: osx
-      osx_image: xcode8.1
+      osx_image: xcode8.3
       rvm: system
     - os: linux
       rvm: 2.0.0

Library/Homebrew/test/.bundle/config

 ---
 BUNDLE_PATH: "../vendor/bundle"
 BUNDLE_DISABLE_SHARED_GEMS: "true"
+BUNDLE_BIN: "../bin"