TravisCI: caching bundler to speed up building

.travis.yml

@@ -1,13 +1,21 @@
 sudo: false
+
+cache: bundler
+
 language: ruby
+
 rvm:
 - 2.2.2
+
 gemfile:
 - Gemfile
+
 script:
 - make check
+
 after_success:
 - bash scripts/build.sh
+
 env:
   global:
   - secure: ewQMcFd3wH8IHOeGhIkBr2kAgadONmesv2KB+9bPRCZ5gvtFFwlkCMdPnQBR9Kd3GKC8r+EO0JNNFWOvrDjPYhEHi9Ab26Q/qodwfwne9YloKXT+C48Zrbrj8q7kb/FeMYPev+RMuM96j5E5QyqBVohGP7hC2bpU3mvAhQ2wBCE=