Fix the OS/2 font tables to prevent clipping on windows

The OS/2 values in the generated fonts were incorrect, so this corrects them. To
do this, I added some files to the khan/MathJax-dev repo, and updated the
dockerfile to use the new repo.

Test Plan:
- View the test page with chrome and firefox on linux, chrome and safari on mac,
  and IE8 and IE9 on windows, and see that none of the characters are clipped.

Auditors: alpert