tap-new: cleanup and update Travis configuration.

- Use macOS Sierra 10.12 and Xcode 8.
- Add indentation.

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

@@ -46,8 +46,8 @@ module Homebrew
     travis = <<-EOS.undent
       language: ruby
       os: osx
-    env: OSX=10.11
-    osx_image: xcode7.3
+      env: OSX=10.12
+      osx_image: xcode8.1
       rvm: system
 
       before_install:
@@ -57,7 +57,6 @@ module Homebrew
           fi
         - sudo chown -R $USER "$(brew --repo)"
         - git -C "$(brew --repo)" reset --hard origin/master
-      - git -C "$(brew --repo)" clean -qxdff
         - brew update || brew update
         - rm -rf "$(brew --repo $TRAVIS_REPO_SLUG)"
         - mkdir -p "$(brew --repo $TRAVIS_REPO_SLUG)"