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