chore: add env for go1.12

.travis.yml

@@ -9,13 +9,15 @@ go:
   - 1.10.x
   - 1.11.x
   - 1.12.x
-  - tip
+  - master
 
 matrix:
   fast_finish: true
   include:
   - go: 1.11.x
     env: GO111MODULE=on
+  - go: 1.12.x
+    env: GO111MODULE=on
 
 install:
   - if [[ "${GO111MODULE}" = "on" ]]; then go mod download; else go get -t -v ./...; fi