Removed NPMLOG.on(), as npmlog is not an EventEmitter now.
Updated npm for travis, as it's own version was breaking builds.
addLevel() can take an additional optional parameter, the stream
to write the logs to, for every level. process.stderr for error
and critical by default.
Coverage increased (+0.7%) to 98.276% when pulling ba97dc0e3276c535a601034ed444cba0c5881ef5 on forkedNpmlog into c34a3cb9c2a6dad583fbfb663a8e1aebcfebde0d on master.