GitLab

As discussed in http://goo.gl/RpYs8e, we have removed
the ability to save dependencies without copying their
source code. This is the last step in the process.

As discussed in http://goo.gl/RpYs8e, we will be
removing the ability to read dependencies without
source code. Warn users now of this upcoming change.

This changes the default behavior of 'godep save' to
preserve any dependencies already in the list with their
existing revision. New dependencies will be added, and
old dependencies will be removed, but that's all. This
makes the save command roughly analogous to 'go get'
with no flags.

fixes #65