Revert "Express is recommended for Stitch."

This reverts commit f205c5680cfdd5dd7d128bcc816dcf7037b6b34b.

Express isn't needed unless the stitch output is meant to be accessible
over HTTP, and we don't need that here.
Define library version.
Initial spike of working.

This design differs pretty radically from the prototype in that it
doesn't depend on any state between invocations of ExecJS::Context#call,
which is actually the recommended way to use ExecJS contexts (see e.g.
[1], [2]).  We can still avoid re-parsing all of JSV and the shim,
though.

[1]: https://github.com/sstephenson/execjs/issues/28
[2]: https://github.com/sstephenson/execjs/issues/53
Travis build configuration.

I'll put this on NUBIC's CI server too, but it's a lot easier to run
tests against multiple ExecJS runtimes on Travis.