build: remove GO15VENDOREXPERIMENT flag as go1.5 has been dropped

.travis.yml

@@ -9,5 +9,4 @@ install:
   - glide install
 script:
   - export PATH=$PATH:$HOME/gopath/bin
-  - export GO15VENDOREXPERIMENT=1
-  - go test -v $(GO15VENDOREXPERIMENT=1 go list ./... | grep -v '/vendor/')
+  - go test -v $(go list ./... | grep -v '/vendor/')