added rvmrc

***Do not merge until this has been reviewed

Hi devs,

I was wondering if it is a good idea to add an rvmrc file to the markus
repository. You can read about them here: https://rvm.io/workflow/rvmrc/

but basically this is what they do: We create a .rvmrc file in the
repository and put in it:

rvm --create use 1.8.7@markus

when people pull this file into their repo and cd into the markus
directory, three things can happen:

1. If you don't have rvm installed, nothing happens
2. If you have rvm installed, rvm automatically checks out ruby 1.8.7
and the gemset markus
3. If you don't have ruby 1.8.7 installed or the @markus gemset, rvm
will prompt you to install ruby and create the gemset

So people don't have to worry about checking out the right version of
ruby to run markus, and the gems are installed at a separate gemset to
avoid conflicts. Not sure if this will make the env installation easier
or will add complexity. Thoughts? 

Mina