Mac builds are broken on Travis

[jameslamb]

See, for example, https://travis-ci.org/uptake/pkgnet/jobs/596707485

Seems our Mac OS builds are failing because newer versions of data.table require libomp? Or at least try to use it by default?

I'm unsure (writing this issue somewhat hastily).

It's possibly that this could be automagically fixed by adding a brew install libomp to our setup or by pegging to an older data.table version. If neither of those works, we can try using a different compiler that has it bundled, so something like:

brew install gcc
export CXX=/usr/local/bin/g++
export CC=/usr/local/bin/gcc

[jameslamb]

this issue is blocking #243 #248 and #249

[jayqi]

Looks like this is the fix: Rdatatable/data.table#3938 (comment)