Add support for target version validation with error/warning messaging
Refactor to collate warnings whilst validating rather than logging directly
Merge branch 'master' into stef/APPS_739_validate_versioning

Conflicts:
	lib/zendesk_apps_support/validations/manifest.rb
Remove parentheses
Move warnings to initialize
Fix spec expectation
Merge pull request #22 from zendesk/stef/APPS_739_validate_versioning

[APPS-739] - As a ZAT user, I want ZAT to ensure my app targets a valid version of the framework