Added Shellout files on the Makefile

[f-meloni]

If you try to run danger-swift pr $PR and you are using the installed swift-danger (then you don't have the full .build folder) you get this error.

Danger: ⅹ Failing the build, there is 1 fail.

Dangerfile.swift:4:8: error: missing required module 'ShellOut'
import Danger
       ^

Not sure this is the best solution to the problem, but adding this and calling make install solved the problem in my case.

[f-meloni]

Looks is having the same problem of #89

[orta]

Interesting, I added some more debug logs to this repo and it's a tad worrying that it just doesn't echo anything :-/