Update rest-client to v2.0

[thibaultdalban]

Just an update of the rest-client gem to use the last version.
Earlier Ruby versions such as 1.8.7, 1.9.2, and 1.9.3 are no longer supported in this version, but the rspotify gem requires a Ruby version '>= 2.0.0'

[guilhermesad]

@thibaultdalban thanks for contributing! However this rest-client version indirectly requires ruby >= 2.2.2

Gem::InstallError: rack requires Ruby version >= 2.2.2

So users with ruby < 2.2.2 would have issues installing the bundle. For that I rather keep it locked in 1.8 until (if) we decide to update the ruby version too.

[thibaultdalban]

@guilhermesad thanks for your feedback !
What about something using an optimistic lock version:
spec.add_dependency 'rest-client', '>= 1.8'
Or with an upper bound to keep the control on future rest-client releases.
spec.add_dependency 'rest-client', ['>= 1.8', '< 2.1']

Let me know if this solution could be acceptable and I will update the PR.

[guilhermesad]

I believe with that the bundler would by default still try to get the latest version, hence causing the headache for some. Is there a reason in specific you want to use latest rest-client?

[thibaultdalban]

Ok I see.
I'm stuck by another gem depending on rest-client 2.0, but I found a workaround.
So I think we can definitely close this PR !
Thank you for your time and I hope my next contribution will be more valuable 😄

++

[guilhermesad]

Sure no worries at all! Appreciate it, thanks