Copy default files over before cleaning sites directory.

- Fixes issue #45
Merge pull request #46 from jhedstrom/copy-defaults

Copy default files over before cleaning sites directory.
Reverting change from 41339b, and implementing yet another workaround to support loading Grunt plugins installed in grunt-drupal-tasks/node_modules. The previous approach caused a regression where the Composer and Bundler tasks did not find composer.json and Gemfile due to the working directory change. This restricts the directory change to the load of external plugins.
Merge pull request #50 from phase2/bug/fix_composer_file_check

Changing approach for loading Grunt plugins
Removing before_script command to remove composer.json.
Extending build test to ensure the vendor directory is created to check composer.json is installed.