add back in tests on latest go, but allow it to fail.

this is an important early warning sign, and I think it's good to add it back in
in a way that doesn't cause the build to fail.

.travis.yml

@@ -8,6 +8,7 @@ language: go
 go:
   - 1.9.x
   - 1.10.x
+  - master
 
 install:
   - make deps
@@ -20,4 +21,6 @@ branches:
     - master
 
 matrix:
+  allow_failures:
+    - go: master
   fast_finish: true