Travis build on OSX

[lekshmideepu]

Added the best possible maximal build for OSX Mojave which fixes #639
MUSIC is turned OFF due to the non-availability of MPI-CXX bindings in Openmpi4.0.1 (Refer INCF/MUSIC#27 (comment)).
Fixes the incorrect nest.init call #1181 and module import error mentioned in #1180

[jougs]

Many thanks providing this extremely useful addition. I have a more structural change request and a suggestion for the wording of a comment. Other than that I'm happy to approve once the Travis run is finished.

[jougs]

I observe exactly the same hang of the MPI testsuite with Open MPI 3.1.3 on Linux. As it sometimes works without problems, I suggest some race condition going on. We really should fix our data exchange in the MPI based testing with SLI. For now, I'm fine with just disabling it here, although I think @heplesser should also have a look.

[jougs]

Many thanks for addressing my comments. I think this really is a huge step forward for us and I'm all in favor of merging it.

[jougs]

@tammoippen, would you be available to review this PR? Thanks!

[tammoippen]

Nice. The gcc selection will break for the next major release, but until then we have some time. Should probably be something like:

grep '^/usr/local/bin/gcc-\d+$

Best.

[jougs]

Awesome. I'm merging.

I'd also like to thank @tammoippen for reviewing @lekshmideepu for putting so much hard work into this - the punishment is over ;-)

[lekshmideepu]

:-) Thanks @jougs

[tammoippen]

I am happy to assist 😄