Pin minimum protobuf dependency to 3.4.0. (#6132)

See: #6128.

setup.py

diff --git a/setup.py b/setup.py
index dbb5102..351c8bf 100644
--- a/setup.py
+++ b/setup.py
@@ -30,7 +30,7 @@
 release_status = 'Development Status :: 5 - Production/Stable'
 dependencies = [
     'googleapis-common-protos<2.0dev,>=1.5.3',
-    'protobuf>=3.0.0',
+    'protobuf>=3.4.0',
     'google-auth<2.0.0dev,>=0.4.0',
     'requests<3.0.0dev,>=2.18.0',
     'setuptools>=34.0.0',