fix panic with unix socket
remove + from minor kernel version
Merge pull request #1041 from unclejack/fix_minor_kernel_version_for_git_kernels

remove + from minor kernel version for kernels built from git
Merge pull request #1035 from dotcloud/fix_panic_cast

- Runtime: fix panic with unix socket
Fix a nil dereference in buildfile_test.go

The test runtime object wasn't properly initialized.
Always return the correct test image.

And not a *random* one from its history.