Introduce go module support

.travis.yml

@@ -5,6 +5,12 @@ go:
   - 1.11.x
   - 1.x
 
+cache:
+  directories:
+    - $HOME/.cache/go-build
+    - $HOME/gopath/pkg/mod
+    - $HOME/Library/Caches/go-build
+
 os:
   - linux
   - osx
@@ -12,6 +18,7 @@ os:
 install:
   - go get -v -d -t ./...
   - npm --prefix client install
+  - go env
 
 script:
   - (cd `go env GOPATH`/src/github.com/prometheus/client_golang && git checkout -q v0.8.0)