CI server on GitHub repos

[Daniel Ferreira (theiostream)]

Hi everyone,

Now that our repos have been migrated to GitHub, it should be trivial
to setup an integrated CI server for GNUstep projects.

For instance, to build our libraries with Travis CI[1] there's barely
any more setup needed than adding a .travis.yml config file to the
project and setting up the gnustep organization over there. Then,
whoever opens pull requests can see seamlessly in the GitHub web UI
whether the pull request broke the project.

Aside from purely passing the tests, it could help new contributors
(like me) to track compatibility goals for libraries like base and
gui. Fred and Richard recently pointed out to me that Base cannot rely
on old machines/compilers having support for address@hidden; and Richard
apparently just had to write a fix about this[2]. If we had a machine
on the CI server to compile base and run its tests with the lowest
version of gcc we intend to support, we'd avoid developers ever having
patches merged with "clangisms" or other features that broke this
compatibility we intend to maintain.

-- Daniel.

[1]: From a brief search in the list, it has already been considered
as an option: 
http://lists.gnu.org/archive/html/gnustep-dev/2017-02/msg00033.html.
[2]: 
https://github.com/gnustep/libs-base/commit/383c3246ee01acfa42a5a8e9d8df3b1b1927f0a7