remove travis.ci badge until we are re-enabled

Change-Id: I5de983828885d5d604dffebc7850efd717342e0b
Reviewed-on: https://gerrit.instructure.com/37130
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>
QA-Review: Brian Palmer <brianp@instructure.com>