[poppler] Example travis configuration file for poppler

[Jeroen Ooms]

We have been working on a .travis configuration file to automatically
test poppler feature branches on various linux and osx configurations.
Perhaps this may be interesting to other poppler users as well.

The example ".travis.yml" file can be copied from:
https://github.com/jeroen/poppler/blob/travis/.travis.yml (example
results: https://travis-ci.org/jeroen/poppler). Github users can
active the CI up by adding the .travis.yml file to the root of the
poppler source directory, and then login to travis.com and activate
the switch for the poppler source repository.

If there is sufficient interest, we can explore additional travis
targets for other platforms, compilers, sanitizers, etc. Large
projects like libcurl [1] and gdal [2] extensively use travis in the
development process. It may be useful to use this for the official
https://github.com/freedesktop/poppler repository.

[1] https://travis-ci.org/curl/curl
[2] https://travis-ci.org/OSGeo/gdal