Loosening version constraint on doctrine/collections
Bumped version to v0.5.1
Merge branch 'release/v0.5.1' into develop

* release/v0.5.1:
  Bumped version to v0.5.1
Merge branch 'develop' of github.com:mcrumm/phlack into develop

* 'develop' of github.com:mcrumm/phlack:
  Upgrade composer requirements to the latest doctrine/collections
  Upgrade composer requirements for doctrine/collections
  Upgrade composer requirements for doctrine/collections

Conflicts:
	CHANGELOG.md
	composer.json