Fix issues with --standalone

When browserify is invoked with --standalone, the bundle() will return
a through2 stream. This stream does not emit the 'close' event. The 'end'
event comes through though.

Switching watchify to use the 'end' event seems to work with and without
--standalone option.

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This would fix issue #36. Although I admit I'm not completely familiar with the difference between the two events. I gave it a quick test and it seemed to work, but not sure if there are cases when the `end` event misbehaves.