v21: Fixes #310

If your fs is case insensitive and your $GOPATH contains a path with a
capital letter (ex. ~/Documents/go), but you work in the lower case
version (ex. ~/documents/go) then godep save will spit out errors about
skipping untracked files.

This is because go list will output the lower case paths, but the
various file inspections godep does end up with upper case names.

So use strings.EqualFold to check across cases.