build: fix maven config errors preventing code generation (#151)
build: fix maven config errors preventing code generation (#151)

Define protobuf.version property needed to bind protoc plugin to verison
of protobuf in use.

Pin fmt-maven-plugin to v2.9 and manually set its version of
google-java-format to 1.7, which is compatible with JDK8.
googleapis/java-conformance-tests,googleapis/java-conformance-tests,googleapis/java-conformance-tests