Use new bundle config instead of --path

Author: adangel

README.rdoc

@@ -113,6 +113,7 @@ The tool creates the following folders:
   git clone https://github.com/pmd/pmd-regression-tester.git
   cd pmd-regression-tester
   gem install bundler
-  bundle install --path=vendor/cache
+  bundle config set path "vendor/cache"
+  bundle install # once
   bundle exec rake verify # run this command before commit your changes
   bundle exec pmdtester ... # run this to directly execute pmdtester from source