Fixed VERSION_SUFFIX logic in update_version.py
Fixed VERSION_SUFFIX logic in update_version.py

This commit makes a couple of fixes:
- Make sure we always update the suffix, since even for a non-RC release
  we need to clear the suffix.
- Make sure the suffix is properly quoted and begins with -rc

google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf,google/protobuf