Bump sinatra, thin, and poltergeist

[girasquid]

Sinatra and Thin so that they can handle Rack 2.0, which Rails 5 requires. Poltergeist so that brew-installed phantomjs works.

@j-mutter @bslobodin

[girasquid]

The test suite...doesn't actually run for me using the updated poltergeist/phantomjs OR the old one; when you install using their README instructions the qless-core that you're testing against has problems like this all the way down:

  126) Qless::ClientEvents can pick up on pop events
     Failure/Error: client.jobs[queue.put(Qless::Job, { foo: 'bar' })]
     Redis::CommandError:
       ERR Error running script (call to f_3f9682e7ddb462dca8c60a26d5e88ac70c3a49e9): @user_script:925: @user_script: 925: Lua redis() command arguments must be strings or integers

[girasquid]

Known issue!. Poking at downgrading my redis and experimenting.

[bslobodin]

So, umm, it took us a while to find courage to review/approve New Relic, how are we going to actually review all these gem changes?

[bslobodin]

beta 😢

[girasquid]

From august https://rubygems.org/gems/sinatra/versions/2.0.0.beta2 😢

[girasquid]

This PR just bumps sinatra + thin instead of dealing with poltergeist at all: #6