Add the Travis script to explicitly test intervening Go versions (#111)

Author: dsnet

.travis.yml

@@ -3,13 +3,21 @@ language: go
 matrix:
   include:
     - go: 1.8.x
-      script: go test -v -race ./...
-    - go: 1.x
+      script:
+        - go test -v -race ./...
+    - go: 1.9.x
+      script:
+        - go test -v -race ./...
+    - go: 1.10.x
+      script:
+        - go test -v -race ./...
+    - go: 1.11.x
       script:
         - diff -u <(echo -n) <(gofmt -d .)
         - go test -v -race ./...
     - go: master
-      script: go test -v -race ./...
+      script:
+        - go test -v -race ./...
   allow_failures:
     - go: master
   fast_finish: true