fix alignment
Merge pull request #6681 from SylvainCorlay/widget_hbox_align

Minor change in widgets.less to fix alignment issue
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
set contentType='application/json'

instead of skipping straight to headers

avoids jQuery's crazy JSONP detection on `=.*??`,
which was triggered because it assumed the contentType was unspecified,
and thus should be guessed based on content.
test saving with problematic cell contents