Bump version number (#1219)
Bump version number (#1219)

Motivation:

We plan on making a release soon.

Modifications:

- Bump the version to 1.2.0

Result:

The version in the default user-agent string will match the released
version.
grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift,grpc/grpc-swift