fix(ci): pre-download go modules before running concurrent jobs

Go modules can run into races when concurrently downloading packages

.travis.yml

@@ -42,6 +42,8 @@ jobs:
         - DESC="Lint Go code"
       before_script:
         - make mock-assets
+        # https://github.com/golang/go/issues/26794
+        - GO111MODULE=on go mod download
       script: make lint-go
 
     - stage: Lint
@@ -75,6 +77,8 @@ jobs:
         - nvm install $(< .nvmrc)
       script:
         - export GO111MODULE=on
+        # https://github.com/golang/go/issues/26794
+        - go mod download
         # compile assets via webpack and build those into bindata_assetfs.go file
         - make bindata_assetfs.go
         # verify that there are no uncommited changes from (re)compiling sass