travis: bump go versions

Bump from 1.8, 1.9 to 1.10.4, 1.11. Pin to specifically 1.10.4 since
otherwise it implies 1.10.0 which has a go-vet bug which causes failures
(see golang/go#24193). Pinning to 1.11.0
doesn't seem to work so leave that as just 1.11.

.travis.yml

@@ -5,8 +5,8 @@ language: go
 go_import_path: github.com/coreos/ignition
 
 go:
-  - 1.8
-  - 1.9
+  - "1.8"
+  - "1.11"
 
 env:
   matrix: