Update README and Travis config with correct package set.

texlive-latex-extra is needed for the etoolbox package, which is required
by polyglossia.

The GNU FreeFont font which we use is included in the texlive-fonts-extra
package.

Also update TeXlive monkey-patch for travis.

Change-Id: I65a084cfc8490a7b9b5c316f6804c83aaf0d66a6
Use texlive backports PPA instead of monkey patching our texlive install.

Change-Id: I633ee7ac4b453bf82fa249685eead80aa28cdc55