Refine .travis.yml

* Disable automatic update and clean up on `brew install` (install time
  160 sec -> 5 sec).
* Avoid using deprecated keys.

.travis.yml

@@ -3,6 +3,9 @@ language: c
 jobs:
   - os: linux
   - os: osx
+    before_install:
+      - HOMEBREW_NO_AUTO_UPDATE=1 HOMEBREW_NO_INSTALL_CLEANUP=1 brew install bison
+      - export PATH="/usr/local/opt/bison/bin:$PATH"
 
 env:
   - MRUBY_TARGET=travis