Update boot2docker

When running boot2docker on my Mac, I ran into this error:

`"DOCKER_PORT=4243 on localhost is used by an other process!"`

Googling for the error message, I came across [this page (in Japanese)](http://www.akiyan.com/blog/archives/2014/02/osx-docker-with-crashplan.html).  But the thing I noticed is the mention of CrashPlan.  I'm also running CrashPlan so it looks like there is a conflict there.

It took me a while to figure out how to deal with this, so I tried to improve the diagnostic message a little bit so other people (after me), could address this a bit more quickly.