change TeX Gyre font package so XeLaTeX can find it

`fonts-texgyre` supersedes earlier `tex-gyre` in including extra material that helps to make fonts findable by XeLaTeX; this PR makes the Travis configuration a better hint about what to install to build our documentation locally on systems like Debian jessie. To test, make sure that the PDF documentation is still properly generated. (Really, Travis is the main tester here, and that seems happy.)

cc @rleigh-dundee