Gopkg fixes (#60)

## Description

* fix: install a release version of dep during builds
* fix: Gopkg.lock issues due to dep update

.circleci/config.yml

@@ -19,7 +19,7 @@ jobs:
     steps:
       - checkout
       - run: go get github.com/mattn/goveralls
-      - run: go get -u github.com/golang/dep/cmd/dep
+      - run: curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
       - run: make build
       - run: make test-integration-cover
       - run: /go/bin/goveralls -coverprofile=coverage-all.out -service=circle-ci -repotoken=$COVERALLS_TOKEN
@@ -30,7 +30,7 @@ jobs:
       - image: circleci/golang:1.9
     steps:
       - checkout
-      - run: go get -u github.com/golang/dep/cmd/dep
+      - run: curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
       - run: make setup
       - setup_remote_docker
       - run: make build_linux
@@ -43,7 +43,7 @@ jobs:
       - image: circleci/golang:1.9
     steps:
       - checkout
-      - run: go get -u github.com/golang/dep/cmd/dep
+      - run: curl https://raw.githubusercontent.com/golang/dep/master/install.sh | sh
       - run: make setup
       - run: curl -sL https://raw.githubusercontent.com/goreleaser/get/master/get | bash
       - setup_remote_docker