Add some real integration specs

Integration specs with a mocked prometheus server.

~~Need to start the mock-prometheus server, `$ ruby spec/support/fauxmetheus/fauxmetheus.rb`~~

~~@juliusv @grobie i'm having issues getting the mock server working on travis, but it's working for me locally. is this working for you guys locally?~~

Mounting `FauxMetheus` (I'm fairly pleased with that name) inside `PromDash` now

Also, travis was having issues running the tests in firefox, so i switched to semaphore (i'm using semaphore for CI in another app). no issues getting the integration specs to pass, and it seems to run faster, too.

and i just noticed that it's not free -.- https://semaphoreapp.com/subscription

I also got the build working on drone.io (which is free), it just doesn't have those nice check marks. Take a look at semaphore and let me know if you guys are interested in getting a subscription, otherwise drone is fine. https://drone.io/github.com/prometheus/promdash/79

also addresses #121