The current docker installer attempt to install the linux-image-extra-`uname -r` DEB package on Debian systems.
This package may not exist, for example on custom kernels (e.g., Linode servers).
If this happens, Vagrant halts the provisioning. However, this package is not really needed in newer Debian releases (such as Ubuntu 14.04).
This small patch checks if the linux-image-extra-`uname -r` package exists, and it will install it if it does. In either case, it will continue provisioning.