remove support for sha to specify rev

[asottile]

this has been producing a warning since may of 2019:

$ pre-commit run flake8 --all-files
[WARNING] Unexpected key(s) present on https://github.com/pre-commit/pre-commit-hooks: sha
flake8...................................................................Passed

this now produces this error:

$ pre-commit run flake8 --all-files
An error has occurred: InvalidConfigError: 
==> File .pre-commit-config.yaml
==> At Config()
==> At key: repos
==> At Repository(repo='https://github.com/pre-commit/pre-commit-hooks')
=====> Missing required key: rev
Check the log at /home/asottile/.cache/pre-commit/pre-commit.log

this can be fixed automatically by running pre-commit migrate-config:

$ pre-commit migrate-config
Configuration has been migrated.
$ pre-commit run flake8 --all-files
flake8...................................................................Passed