PDF builds change font back to FreeSerif

The pdf build fails if OpenSans fonts are not installed as system fonts.
But there are not any standard packages to install them. Change back to
FreeSerif for now so the pdf can be built on a standard installation.

source/conf-common.py

@@ -203,7 +203,7 @@ latex_logo = 'mahara/static/mahara-latex.png'
 
 latex_use_parts = False
 
-LATEX_MAINFONT = 'OpenSans'
+LATEX_MAINFONT = 'FreeSerif'
 LATEX_BABEL = '\\usepackage{babel}'
 LATEX_TITLE = "Mahara user manual"
 LATEX_AUTHOR = "Catalyst IT and others"