Update utils.go to not enforce extra constraints on the kernel "flavor" (such as being integral or even comparable one to another)

This is especially to fix the current docker on kernels such as gentoo-sources, where the "flavor" is the string "gentoo", and that obviously fails to be converted to an integer.

This was discussed with @shykes in https://github.com/dotcloud/docker/issues/313#issuecomment-16746821.