Remove go get because we use go mod

.travis.yml

@@ -1,5 +1,6 @@
-language: go
+os: linux
 dist: bionic
+language: go
 
 go:
   - 1.15.x
@@ -14,13 +15,6 @@ before_install:
   - sudo DEBIAN_FRONTEND=noninteractive apt-get install -y librados-dev librbd-dev
 
 install:
-  - go get github.com/google/go-cmp
-  - go get github.com/stretchr/testify
-  - go get github.com/sirupsen/logrus
-  - go get github.com/ceph/go-ceph
-  - go get github.com/prometheus/client_golang/prometheus
-  - go get golang.org/x/tools/cmd/cover
-  - go get github.com/axw/gocov/gocov
   - go get github.com/modocache/gover
   - go get github.com/mattn/goveralls