Freeze vcap-services-base at version used by vcap-services/ng/mysql

It seems my previous [PR](https://github.com/cloudfoundry-community/cf-services-contrib/pull/1) did not solved the problem (it introduced an other problem).
 
Specifying [no version](https://github.com/cloudfoundry-community/cf-services-contrib/pull/1) makes the fix time depended as can be seen here. The first time I tested it local it worked. A few day's later when I commited it and created a pull request things must have been changed and the build was broken again.

I now have frozen the vcap-services-base and vcap-common on the refs used by the [mysql-service](https://github.com/cloudfoundry/vcap-services/blob/master/ng/mysql/Gemfile.lock#L11-L28).

Notes to self for how to update gems for services:
change Gemfile
run `bundle install`
run `bundle package` to vendor the gems

bosh create release
bosh upload release
bosh deploy