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
Mention rebinding behavior of +=

Explicitly mention "rebind", hopefully facilitates search
Ref: https://groups.google.com/d/msg/julia-users/ujxesTieDHU/zWrZNAUHelkJ
Merge pull request #8744 from JuliaLang/tk/default-travis-multi-os

Use language: julia in default Travis
get versioninfo() from AppVeyor

[ci skip]
Really fix #9366

pass a stack-copy of ctxThread in jl_throw_in_ctx to rec_backtrace_ctx
trim the stack back down to the bare necessity
run win64 in parallel again on appveyor

let's see if maybe we just fixed the cause of the freezes
[ci skip]
add graphemes(s) function to iterate over graphemes (represented by substrings) of a string s