Merge pull request #27 from kwonoj/pin-protobuf

fix(protobuf): pin down protobuf to 3.5.2

Author: kwonoj

.travis.yml

@@ -8,7 +8,7 @@ env:
   global:
     - DOCKER_REPO='ojkwon/arch-emscripten'
     - EMSCRIPTEN_VERSION=1.38.13-1
-    - PROTOBUF_VERSION=3.6.1
+    - PROTOBUF_VERSION=3.5.2
   matrix:
     - BUILD_TARGET='base'
     - BUILD_TARGET='protobuf'