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 )
add git fetch --unshallow in commented out script section

add explanatory comment
turn on coverage in commented-out script section
Merge pull request #8744 from JuliaLang/tk/default-travis-multi-os

Use language: julia in default Travis