Post PR #4559: add back `.dev1` to version numbers. (#4561)

diff --git a/setup.py b/setup.py
index d07393c..ede41b1 100644
--- a/setup.py
+++ b/setup.py
@@ -68,7 +68,7 @@
 
 setup(
     name='google-api-core',
-    version='0.1.2',
+    version='0.1.3.dev1',
     description='Core Google API Client Library',
     long_description=README,
     namespace_packages=['google'],