fix: golang/protobuf version

`github.com/golang/protobuf`:
- `github.com/prometheus/client_golang` (no version)
- `github.com/gogo/protobuf` (no version)
- `google.golang.org/appengine` (no version)
- `github.com/matttproud/golang_protobuf_extensions` (no version)

State:
- Current version: 2bba0603135d7d7f5cb73b2125beeda19c09f4ef
- Glide suggest: 8616e8ee5e20a1704615e6c8d7afcdac06087a67

Force to keep the current version.

Refs
- 2bba060313 (Mar 31, 2017) next commit the Apr 27, 2017.
- 8616e8ee5e (8 Jun 2016)

glide.yaml

@@ -156,6 +156,8 @@ import:
   version: v0.3
   subpackages:
   - proto
+- package: github.com/golang/protobuf
+  version: 2bba0603135d7d7f5cb73b2125beeda19c09f4ef
 - package: github.com/rancher/go-rancher
   version: 5b8f6cc26b355ba03d7611fce3844155b7baf05b
 - package: golang.org/x/oauth2/google