Update gems

- puma: 1.6.3 -> 2.4.0
- sinatra: 1.3.5 -> 1.4.3
- slim: 1.3.6 -> 2.0.0
- twitter: 4.6.0 -> 4.8.1
- omniauth: 1.1.3 -> 1.1.4
- omniauth-twitter: 0.0.14 -> 1.0.0
Update development gems

- better_errors: 0.7.2 -> 0.9.0
- binding_of_caller: 0.7.1 -> 0.7.2
Explicitly use Ruby 1.9.3
Remove deleted users