[FIXUP][.travis.yml] Don't use bundle exec

Since we are skipping `bundle install`, don't depend on it when
launching the rake tasks.

Author: NickLaMuro

.travis.yml

@@ -15,7 +15,7 @@ env:
     - RVM_RUBY_VERSION=ruby-head
 
 install:
-- bundle exec rake test:docker:$RVM_RUBY_VERSION:build
+- rake test:docker:$RVM_RUBY_VERSION:build
 
 script:
-- bundle exec rake test:docker:$RVM_RUBY_VERSION
+- rake test:docker:$RVM_RUBY_VERSION