Cirq v0.8.1

This point release fixes method signatures in documentation and does not change any user-facing functionality.

No other changes to cirq functionality. No protocol changes, top-level changes or other changes.

Complete list of changes:

6dd7484 Fix methods missing from doc strings (#3048)
0567d33 Change version to 0.8.1.dev on 0.8.1 release branch
3fded88 Remove dev version on release branch