Change version to 0.8.1.dev on 0.8.1 release branch

Author: dstrain115

cirq/_version.py

@@ -28,4 +28,4 @@
         'B) Explicitly install an older deprecated-but-compatible version '
         'of cirq (e.g. "python -m pip install cirq==0.5.*")')
 
-__version__ = "0.8.0"
+__version__ = "0.8.1.dev"