v0.2.1

• The Requirements section of both READMEs said rasuvaeff/understudy ^0.1
  while composer.json has required ^0.4 since 0.2.0.
• Allow rasuvaeff/understudy ^0.5. Widened rather than raised: the
  extension works against both, and 0.4 consumers should not be cut off from
  it.