Travis: don't install all the *-doc packages

This should significantly improve download time / traffic.
Merge branch 'master' into travis

Conflicts:
	.travis.yml
Travis: fix the missing "lmroman10" problem

I followed the advice from:

http://tex.stackexchange.com/questions/652/xetex-not-working-on-ubuntu-10-04
Re-enable matplotlib printing in init_printing

Re-enable matplotlib as a possible way to png render latex. This
addresses some problems with Windows where using external rendering is
slow and flashes up batch windows. This option is specified through
use_latex by passing 'matplotlib'.
Merge pull request #2272 from certik/travis

Travis: don't install all the *-doc packages
Merge pull request #2299 from flacjacket/matplotlib_printing

Re-enable matplotlib printing as option for init_printing