Bump version to 4.0.1

* Locks down version of cookiejar to <= 0.3.0: The 'em-http-request'
dependency uses this gem, but is apparently incompatible with version
0.3.1 of cookiejar. The em-http-request owners have locked this
dependency down in recent commits, but have not yet published that
change to rubygems, so we have to lock cookiejar manually until they do.