use caching for faster Travis builds

Author: SethTisue

.travis.yml

@@ -3,6 +3,10 @@ sudo: false
 
 language: ruby
 
+# for faster builds
+cache: bundler
+env: BUNDLE_PATH=bundle-vendor/bundle
+
 # we must override install, or Travis's default Gemfile support
 # will kick in and use `vendor`, ignoring our BUNDLE_PATH
 # declaration in `.bundle/config`