Support old IE in airtap client code.

[goto-bus-stop]

The simplest alternative to #273.

This makes airtap's tape client code work in IE9 (and maybe earlier,
buffer@4 actually works all the way back to IE6 afaik).

This only applies to airtap's own code so if old IE support is required
for a project's browser tests, they still need to do the config thing:

browserify:
- require: 'buffer/'
  expose: 'buffer'

So projects can still use the new buffer implementation, airtap itself
will just be using the old one for maximum compatibility.

Closes #273

[vweevers]

Is there any risk / gotchas to potentially having 2 semi-global versions of buffer in the client?

[goto-bus-stop]

I don't think so. They don't affect the global object. The only risk would be if a Buffer was somehow sent from one bundle to the other, and the other bundle used some functionality that is not supported by that Buffer. But all common operations are available on both, and the Buffer.isBuffer() function is provided by is-buffer which works with both versions. AFAIK buffers are never passed around like that in airtap in the first place, so even if there was a difference it prob wouldn't matter 😄

[vweevers]

@goto-bus-stop Alright if we let this sit for a few days, or are you blocked by it?

[goto-bus-stop]

Alright even if it sits for a very long time, no hurry

[vweevers]

I'm gonna explore #57 for a little bit, then we wouldn't need buffer at all and can also use the latest tap-parser in the backend.