Travis CI Integration

[JoostJM]

Despite updates in #643, Travis CI still reports errors during testing.
This PR implements a further fix to the Travis CI Pipeline, by updating Brew before installing gettext.

[JoostJM]

@jcfr, Have you encountered further similar issues with building on Travis CI after your fix in scikit-build?
I find it provides some unexpected and difficult to reproduce behaviour (e.g. 2 branches that are completely identical, save for the branch name, but in one branch the Python 3.7.8 build fails on updating brew, whereas on the other branch it succeeds

[JoostJM]

After restarting the failed job, it succeeded. Going from the log of a similar failed build, it appears the issue may be due to the homebrew repository being unavailable at times.

[jcfr]

Have you encountered further similar issues with building on Travis CI after your fix in scikit-build?
Python 3.7.8 build fails on updating brew

Indeed, see scikit-build/cmake-python-distributions#113

[JoostJM]

Indeed, see scikit-build/cmake-python-distributions#113

I've indeed found this PR, and updated the travis.yml accordingly. It seem to work no most of the time. Did you encounter any connection issues to the homebrew repository lately?

[jcfr]

Did you encounter any connection issues to the homebrew repository lately?

No

Look like the issue is the connection to GitHub ....

fatal: unable to access 'https://github.com/Homebrew/homebrew-core/': transfer closed with outstanding read data remaining

I am not sure what is happening, GitHub status does not indicate anything: https://www.githubstatus.com/history

Searching on internet for transfer closed with outstanding read data remaining + Travis may give some hints. I didn;t have a chance to look into more details.

[JoostJM]

Found a related Stackoverflow thread.

For now, merging this branch. If issues continue to arise, I will try to implement a manual install of homebrew via updating the install.sh to use ssh connection.