Add basic Travis-CI .travis.yml, to let Travis CI run automated build tests.

Change-Id: Iceae442c13f30b57842b300c0920108b614c75f7
Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Reviewed-on: http://openocd.zylin.com/5414


Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>