Add specific Ruby version

I added the ruby version so that services like packager.io can use this information to build packages according to our dependencies. Version 2.1.3 is the current stable so I think we should use this version. 

We recommend 2.1.0 on our [system requirements](https://community.openproject.org/projects/openproject/wiki/systemrequirements) page on OpenProject. So we might want to use this version instead?