Change version to 1.15.0 (grpc#2301)

[skip ci] Skipping Travis. Version number change only

Change version to 1.14.0 (grpc#2245)

[skip ci] Skipping Travis. Version number change only

Change version to 1.13.0

Change version to 1.12.2

Change version to 1.12.1

Change version to 1.12.0

Change version to 1.11.3

Change version to 1.11.2

Change version to 1.11.1

Change version to 1.10.1