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
Rewrite atan2 as arg function
Improve rewrite of atan2 in terms of arg
Test rewrite to arg
Eval conjugates of atan2 expressions
Address review comments
Merge pull request #2285 from raoulb/atan2_improve

Atan2 improve
Add description for kwarg 'show' for plot()
Minor rewording of the kwarg 'show' description for plot()
Minor changes to documentation
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
Merge pull request #2282 from Manoj-Kumar-S/sphinx

Minor changes to sphinx documentation
Added docs for logic module
Modified simplify_logic docs
Merge pull request #2304 from srjoglekar246/logicdocs

Added Sphinx docs for logic module
show kwarg for plot(): some more minor tweaks to the description