upgrade MathJax to version 2.2

This pull request upgrades MathJax to version 2.2. As with the old version, I did not include the png fonts, since they take up a lot of space and we don't use them. I also deleted the file .gitignore from data/mathjax, since it was from the MathJax repository, not ours. I removed some lines from data/sage/html/notebook/base.html which had fixed a bug in an old version of MathJax. That bug has now been fixed in MathJax itself.