Update installation.rst
Update installation.rst

Fixed: github suggests https:// instead of git://
mitya57/mathjax-docs,mathjax/MathJax-docs,Axure/MathJax-docs,GerHobbelt/MathJax-docs,mathjax/MathJax-docs,Axure/MathJax-docs,mitya57/mathjax-docs,GerHobbelt/MathJax-docs