Bump version of gRPC again so that "bazel test ..." works again. (baz…

…elbuild#80)

The error was:

```
ERROR: [...]/external/googleapis/BUILD.bazel:70:1: Generating Descriptor Set proto_library @googleapis//:_google_rpc_status_cc_proto_only failed (Exit 1) protoc failed: error executing command bazel-out/host/bin/external/com_google_protobuf/protoc '--proto_path=external/googleapis' ... (remaining 6 argument(s) skipped)

Use --sandbox_debug to see verbose messages from the sandbox
google/protobuf/any.proto: File not found.
google/rpc/status.proto: Import "google/protobuf/any.proto" was not found or had errors.
google/rpc/status.proto:91:12: "google.protobuf.Any" is not defined.
```

I think this was caused by my previous bump of the gRPC version.

WORKSPACE

@@ -33,12 +33,12 @@ http_archive(
 # Needed for C++ gRPC.
 http_archive(
     name = "com_github_grpc_grpc",
-    strip_prefix = "grpc-1.20.1",
+    strip_prefix = "grpc-1.21.0",
     urls = [
-        "https://github.com/grpc/grpc/archive/v1.20.1.tar.gz",
-        "https://mirror.bazel.build/github.com/grpc/grpc/archive/v1.20.1.tar.gz",
+        "https://github.com/grpc/grpc/archive/v1.21.0.tar.gz",
+        "https://mirror.bazel.build/github.com/grpc/grpc/archive/v1.21.0.tar.gz",
     ],
-    sha256 = "ba8b08a697b66e14af35da07753583cf32ff3d14dcd768f91b1bbe2e6c07c349",
+    sha256 = "8da7f32cc8978010d2060d740362748441b81a34e5425e108596d3fcd63a97f2",
 )
 
 # Pull in all gRPC dependencies.