Use login shell to install gems

When running this role, I found that my rbenv did not actually install any gems. A bit of googling suggested that using bash -lc was a more reliable way of installing gems into rbenv so I've had a go with this, and it seems to work.

Have you had any trouble with this? If so, this seems to be a fix. If not, I'd love to know if the gem module works correctly for you!

Thanks for writing the role, I've found it really useful.