Update go.mod to go 1.15

The version in go.mod is used by `go build` to select the version
of the Go language to target.  If it is older than the version of Go
in the tree then command line tools like `go test` will give different
results from inside the build.  Update the version to match the version
of Go in the tree.

Test: go build ./java
Change-Id: I418f401580be7696e2829c970938c7c82715b932