Build the docs on Travis with docutils 0.12
Build the docs on Travis with docutils 0.12

0.13.1 has a bug that prevents the build from working.

Fixes #11930.

jaimahajan1997/sympy,kaushik94/sympy,kaushik94/sympy,jaimahajan1997/sympy,yashsharan/sympy,skidzo/sympy,kaushik94/sympy,skidzo/sympy,yashsharan/sympy,madan96/sympy,chaffra/sympy,jaimahajan1997/sympy,madan96/sympy,madan96/sympy,chaffra/sympy,yashsharan/sympy,skidzo/sympy,chaffra/sympy