Use 'go tip' version to fix travis build failure

.travis.yml

@@ -4,6 +4,6 @@ go:
   # Used for building XenServer Dundee
   - 1.4.2
   # Forwards compatibility
-  - release
+  - tip
 script:
   - gofmtresult=$(gofmt -s -l .); if [[ -n $gofmtresult ]]; then echo -e "Please run \"gofmt -s -w .\" before committing for the below:\n$gofmtresult"; false; fi