Travis CI tweaks

Tweak our Travis CI configuration and the one we suggest for taps to
be faster and use caching.

.travis.yml

@@ -8,9 +8,11 @@ matrix:
       osx_image: xcode8.3
       rvm: system
     - os: linux
+      sudo: false
       rvm: 2.0.0
 
 before_install:
+  - rm -rf Library/Homebrew/vendor/bundle/
   - export GEM_HOME="$PWD/Library/Homebrew/vendor/ruby/2.0.0"
   - gem list --installed bundler || gem install bundler
   - export PATH="$GEM_HOME/bin:$PATH"

Library/Homebrew/dev-cmd/tap-new.rb

@@ -46,9 +46,10 @@ module Homebrew
     travis = <<-EOS.undent
       language: ruby
       os: osx
-      env: OSX=10.12
       osx_image: xcode8.3
       rvm: system
+      cache:
+        directories: /usr/local/Homebrew/Library/Homebrew/vendor
 
       before_install:
         - export TRAVIS_COMMIT="$(git rev-parse --verify -q HEAD)"