Upgrade when module to improve IE10+ compat

When testing the browserify build for issue #6, I noticed a strange
error thrown when making the initial websocket connection. I managed to
track it down to an IE 'feature' that crops up when you alias in-built
functions. In particular, the when module was aliasing setImmediate to
an internal function (nextTick.) In a newer version of when, the
function is instead aliased to the browserify process.nextTick. This
works well because substack already had that covered.

With when@2.7.0, IE11 appears to be working well. IE10 is still pending
a test.