fix param types on methods in _EmptyVersion

[devoncarew]

Fix two type issues found by ddc - the params of two methods differed from what was defined in the parent class:

Invalid override. The type of _EmptyVersion.allowsAll ((Version) → bool) is not a subtype of VersionConstraint.allowsAll ((VersionConstraint) → bool). (package:pub_semver/src/version_constraint.dart, line 212, col 3)
Invalid override. The type of _EmptyVersion.allowsAny ((Version) → bool) is not a subtype of VersionConstraint.allowsAny ((VersionConstraint) → bool). (package:pub_semver/src/version_constraint.dart, line 213, col 3)

(see also dart-atom/dart#327)

[nex3]

Can you add a CHANGELOG entry and bump the pubspec version?

[devoncarew]

Sure - rev'd from 1.2.1 to 1.2.2 - will squash the commits.