fix(dependencies): increase protobuf version (#49)

Increase protobuf version to 3.12.0 for `_internal_create_key`. Closes #48

Author: busunkim96

setup.py

@@ -30,7 +30,7 @@
 release_status = "Development Status :: 5 - Production/Stable"
 dependencies = [
     "googleapis-common-protos >= 1.6.0, < 2.0dev",
-    "protobuf >= 3.4.0",
+    "protobuf >= 3.12.0",
     "google-auth >= 1.14.0, < 2.0dev",
     "requests >= 2.18.0, < 3.0.0dev",
     "setuptools >= 34.0.0",