Fix deprecation warning for Bundler.with_clean_env

Fixes the following:

    [DEPRECATED] `Bundler.with_clean_env` has been deprecated in favor
    of `Bundler.with_unbundled_env`. If you instead want the environment
    before bundler was originally loaded, use `Bundler.with_original_env`
    (called at /Users/x/src/overcommit/spec/integration/gemfile_option_spec.rb:63)

Author: sds

spec/integration/gemfile_option_spec.rb

@@ -60,7 +60,7 @@ def run
     repo do
       # Since RSpec is being run within a Bundler context we need to clear it
       # in order to not taint the test
-      Bundler.with_clean_env do
+      Bundler.with_unbundled_env do
         FileUtils.mkdir_p(File.join(fake_gem_path, 'lib'))
         echo(gemspec, File.join(fake_gem_path, 'my_fake_gem.gemspec'))
         touch(File.join(fake_gem_path, 'lib', 'my_fake_gem.rb'))