internal/goversion: update to 1.15

[dmitshur]

goversion.Version is currently 14 on master branch (also see CL 230024 and CL 230029).

It needs to be incremented to 15 for the Go 1.15 release.

[gopherbot]

Change https://golang.org/cl/230382 mentions this issue: internal/goversion: update to 1.15