travis: explicitly enable go modules in Go 1.11 and 1.12

.travis.yml

@@ -19,6 +19,8 @@ jobs:
       os: linux
       dist: xenial
       go: 1.11.x
+      env:
+        - GO111MODULE=on
       script:
         - go run build/ci.go install
         - go run build/ci.go test -coverage $TEST_PACKAGES
@@ -27,6 +29,8 @@ jobs:
       os: linux
       dist: xenial
       go: 1.12.x
+      env:
+        - GO111MODULE=on
       script:
         - go run build/ci.go install
         - go run build/ci.go test -coverage $TEST_PACKAGES