Revert "Remove Travis badge from README"

Now that the Travis build is green again, we can add the badge back to
the README as it is now a useful signal.

This reverts commit 3ae530e27df6c2b3d6455f13b67c7d6d0ca6cb0d.

Change-Id: I24c4893cd7b2685324d0a3ef280a26f177b90d8c
Reviewed-on: http://gerrit.causes.com/35878
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>