Travis-CI: revert Firefox testing.
Travis-CI: revert Firefox testing.

Firefox in Travis-CI hangs for some reason:
https://travis-ci.org/dchest/tweetnacl-js/builds/29074712

It works OK on my machine.

Disable Firefox testing for now, all further experiments to figure out
how to fix Travis should go to a branch.

dchest/tweetnacl-js,dchest/tweetnacl-js,dchest/tweetnacl-js,dchest/tweetnacl-js