install local mathjax on Travis

when running the js tests

I'm not sure why loading MathJax from the CDN is falling on Travis,
but it's not happening locally.
Merge pull request #6703 from minrk/install-mathjax-travis

install local mathjax on Travis
require less >= 1.7.5

fixes bug where `vbox > *` style was applied to `vbox()` classes