Allow semicolum as a delimiter for rpc definitions

Google canonical protos always include a comma after each definition. The following service definition was not supported, this PR fixes it.

~~~
service DatastoreService {
  // Look up some entities by key.
  rpc Lookup(LookupRequest) returns (LookupResponse) {
  };
  // Query for entities.
  rpc RunQuery(RunQueryRequest) returns (RunQueryResponse) {
  };
  // Begin a new transaction.
  rpc BeginTransaction(BeginTransactionRequest) returns (BeginTransactionResponse) {
  };
  // Commit a transaction, optionally creating, deleting or modifying some
  // entities.
  rpc Commit(CommitRequest) returns (CommitResponse) {
  };
  // Roll back a transaction.
  rpc Rollback(RollbackRequest) returns (RollbackResponse) {
  };
  // Allocate IDs for incomplete keys (useful for referencing an entity before
  // it is inserted).
  rpc AllocateIds(AllocateIdsRequest) returns (AllocateIdsResponse) {
  };
}
~~~