Declare protobuf as a dependency (#59)

Author: michael-booth

setup.py

@@ -12,7 +12,8 @@
                       'six',
                       'basictracer>=3.0,<3.1',
                       'googleapis-common-protos==1.5.3',
-                      'requests>=2.19,<3.0'],
+                      'requests>=2.19,<3.0',
+                      'protobuf>=3.6.0,<4.0'],
     tests_require=['pytest',
                    'sphinx',
                    'sphinx-epytext'],