Migrate to dep for dependency management (#26)

* Replace `Godeps` with `dep`
* Upgrade all dependencies
* Remove vendor source from git repository

docker-compose.override.yml

@@ -0,0 +1,7 @@
+# this file is used to mount the current directory as a volume
+# we remove it in CI to use files inside the container only
+version: '2.3'
+services:
+  dbmate:
+    volumes:
+      - .:/go/src/github.com/amacneil/dbmate