Add kernel version check

_NOTE: You will need to satisfy kernel requirements for Docker._ It's currently recommended to use 3.10, although 3.8 works as well. We have done some additional testing on Ubuntu Preciese, and found that the oldest kernel that Ubuntu ships and weave happily runs on is 3.5, presumably some some of the issues had been fix since Docker docs were last updated. This is not to say that kernel version is good one to use, we do recommend to use the latest, but just in case if you have any specific reason why you cannot upgrade, then the 3.5 is the oldest we have tried.
s/then/than/

(And yes it should exit)