installation: adapt to new MathJax source URL

• MathJax download place moved to github, so fix download instructions to use another location.

• Still using old version v1.0.1a here; we should upgrade to v1.1. (Or at least install MathJax-Font-Update.zip so that Firefox 3.6.13 and higher would be properly supported with MathJax 1.0.1a.)