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.
Pluggable PRNG.

randombytes can now be overrided with nacl.setPRNG().

Standard PRNG (window.crypto/crypto) initialization moved into
high-level API part, where it calls nacl.setPRNG.