check-yaml rejects files with the %YAML directive

[dunmatt]

When I run pre-commit against a file that has a YAML directive (ie %YAML:1.0 is the first line of the file) it incorrectly tells me:

while scanning a directive
  in "src/example.yaml", line 1, column 1
found unexpected non-alphabetical character
  in "src/example.yaml", line 1, column 6
while scanning a simple key

Expected behavior:
It shouldn't complain, the YAML directive is part of the YAML standard.

[asottile]

please report upstream