Remove Travis badge from README

The Travis environment is really difficult to debug, and currently has
failing tests for Overcommit that aren't reproduceable locally.

Until I have time to figure these out, I don't want what is in my
opinion an incorrect "build failed" badge on our front-facing page.

Change-Id: I0ce896c6a6ceb04900f5b1f88e490fb6e0ad6bfa
Reviewed-on: http://gerrit.causes.com/35642
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>