Update gem version & support, Bundler

[FloorD]

This issue consolidates #92 #89 #87 #60 #50 #22

• Update Gem support
  -- Support gtk3 gem?
• Bump Bundler version to support newer gems
• Addition to the docs for gem maintainers (Guide should describe how to set up a ruby gem #22)

[Nowaker]

@FloorD Great to see some activity in the project! Please have a look at #6. This is the most important ticket related to gems. Most notably, @FooBarWidget's spreadsheet where the community votes for new gems to be supported: https://docs.google.com/spreadsheets/d/1oNgrwHyV9UaMGFTttsfA9QPvL461NWLfHeeAap3Az_A/edit#gid=0 (and where @FooBarWidget rejected gtk3 for being too complex which I agree with).

[starkgate]

Finally some news ! No gtk3 support would be a shame, it's the one thing I'm waiting for.