Switch to using the dep tool and update all the dependencies

CONTRIBUTING.md

@@ -139,14 +139,42 @@ Documentation for rclone sub commands is with their code, eg
 There are separate instructions for making a release in the RELEASE.md
 file.
 
-## Updating the vendor dirctory ##
+## Adding a dependency ##
 
-Do these commands to update the entire build directory to the latest
-version of all the dependencies.  This should be done early in the
-release cycle.  Individual dependencies can be added with `godep get`.
+rclone uses the [dep](https://github.com/golang/dep) tool to manage
+its dependencies.  All code that rclone needs for building is stored
+in the `vendor` directory for perfectly reproducable builds.
 
-  * make build_dep
-  * make update
+The `vendor` directory is entirely managed by the `dep` tool.
+
+To add a new dependency
+
+    dep ensure github.com/pkg/errors
+
+You can add constraints on that package (see the `dep` documentation),
+but don't unless you really need to.
+
+Please check in the changes generated by dep including the `vendor`
+directory and `Godep.toml` and `Godep.locl` in a single commit
+separate from any other code changes.  Watch out for new files in
+`vendor`.
+
+## Updating a dependency ##
+
+If you need to update a dependency then run
+
+    dep ensure -update github.com/pkg/errors
+
+Check in in a single commit as above.
+
+## Updating all the dependencies ##
+
+In order to update all the dependencies then run `make update`.  This
+just runs `dep ensure -update`.  Check in the changes in a single
+commit as above.
+
+This should be done early in the release cycle to pick up new versions
+of packages in time for them to get some testing.
   
 ## Writing a new backend ##