all: Bump protobuf to 3.0.2, to fix protoc in CI

protoc no longer builds in 3.0.0 because auto-download of the gmock zip
now fails. 3.0.2 has a fix to autogen:
https://github.com/google/protobuf/commit/bba446bbf2ac7b0b9923d4eb07d5acd0665a8cf0

All that was strictly necessary was to update .travis.yml and
buildscripts/, but it helps our sanity to keep the rest of the protobuf
versions in sync. Lite is left on its existing version, because it did
not see a bump of neither the java library nor the protoc plugin.
diff --git a/compiler/build.gradle b/compiler/build.gradle
index 7e116d4..c0ffd4a 100644
--- a/compiler/build.gradle
+++ b/compiler/build.gradle
@@ -163,7 +163,7 @@
       if (project.hasProperty('protoc-gen-javalite')) {
         path = project['protoc-gen-javalite']
       } else {
-        artifact = "com.google.protobuf:protoc-gen-javalite:${protobufVersion}"
+        artifact = libraries.protoc_lite
       }
     }
     grpc {