BLD: Install texlive packages in Travis CI
- Closes #12543
- `texlive-latex-base` needed for `latex`
- `texlive-latex-extra` needed for `utf8x.def`

@evectant Thanks! 
I am going to merge for now, but would want to test the mathjax extension in the future (only drawback of this that this will not work when using the docs offline)
 @jorisvandenbossche Sure, just wanted to fix this for now so I could verify the documentation changes in #12492.
 And it is looking good now!
