Backport #8744 (language: julia in default .travis.yml )

Use language: julia in default .travis.yml

and add the os: section for mac testing (has to be explicitly enabled
by sending an email to support@travis-ci.com, see
http://docs.travis-ci.com/user/multi-os )

(cherry picked from commit 6cb77aa0f810c740026ad8a0c33745344fec488e)

add git fetch --unshallow in commented out script section

add explanatory comment

(cherry picked from commit 9b4453a242c99c6e2e70c7f1bd7e351275d81eea)

turn on coverage in commented-out script section
(cherry picked from commit a92ea425bb447c4f70f1656e9ba3c005bce400f5)