add travis-ci badge to README.md

Change-Id: I670fcb90d4faf4529289c15748ae37c6da03c7cc
Reviewed-on: https://gerrit.instructure.com/29014
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>