Permit failures on Go tip

* Try removing `go get .../cover`
* Try using latest goveralls
* Try forcing GO111MODULE=on
* Restore to original, add allow_failures option

.travis.yml

@@ -8,6 +8,10 @@ go:
   - 1.12.x
   - master
 
+matrix:
+  allow_failures:
+    - go: master
+
 before_install:
   # Fetch/build coverage reporting tools
   - go get golang.org/x/tools/cmd/cover