Migrate to dep for dependency management (#26)

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

docker-compose.yml

@@ -4,8 +4,6 @@ services:
     build:
       context: .
       target: build
-    volumes:
-      - .:/go/src/github.com/amacneil/dbmate
     depends_on:
       mysql:
         condition: service_healthy