Updates the Travis CI testing matrix

Change requested by @elithrar to improve the Go versions tested against. 1.x will always point to the latest version. Master points to the latest rev, which is useful for identifying breaking changes in upcoming versions.

.travis.yml

@@ -1,9 +1,13 @@
 language: go
 sudo: false
-go:
-  - 1.7
-  - 1.8
-  - tip
+matrix:
+  include:
+    - go: 1.7
+    - go: 1.8
+    - go: 1.x
+    - go: master
+  allow_failures:
+    - go: master
 script:
   - go get -t -v ./...
   - diff -u <(echo -n) <(gofmt -d -s .)