Deprecate .baseline and migrate to .lastBaseline

Updated Podspec

Updated Changelog

Swift 3.0 Migration

Fix deprecated APIs

Releasing 0.21.0

Merge branch 'develop' of github.com:SnapKit/SnapKit

# Conflicts:
#	ISSUE_TEMPLATE.md

Fix improper revert

Bump versioning for 0.19.0 release

Updated podspec