godep: include dependency source code

Flag -copy=false in godep will be removed. This commit
switches to using 'godep save' with no flag. We don't
change the version of any dependencies here, just rerun
godep to include source code.

See also the proposal for removing flag -copy at
http://goo.gl/RpYs8e.
Merge pull request #36 from godep-migrator/master

godep: switch to -copy=true