Travis: Automatically test KS submissions

With @sircmpwn's permission, this should download and test any mod from
KerbalStuff. This is only triggered on PRs, and will automatically fetch
the latest stable netkan.exe for testing.
Travis/KerbalStuff: Test against the branch we're merging into.
Netkan/KS errors are more verbosely reported.
Travis: Also test github releases.
Skip inflate tests if no .netkan files have changed.
travis: Really mark test as skipped if no .netkan files changed.
Merge pull request #177 from pjf/better_ks_travis

Travis: Test KS and Github releases with netkan.exe