Support non-ci projects

[hasanen]

Problem: I have project without CI and wanted to use herkko for the deploy. Herkko insisted me to install travis.

$ herkko heroku deploy
-> Doing deployment to heroku...
-> Deploying changes:

*Changes* 

-> Checking CI...
-> Travis CLI has not been installed, run: gem install travis
-> CI is in unknown state (unknown). Can't continue.

Desired solution: Herkko would ask (or I could set it) if I have CI in use and then allow me to do deploy. And if possible, it would run tests locally if CI is not set.