Release 1.3.1

See #99, release log:

• 5b5d5ae by @rultor: version 1.3.1
• 9c9aced by @rafamizes: Update pubspec.yaml
• 5ad81ec by @rafamizes: build: update 'flutter_secure_...
• ee57ec4 by @rafamizes: Update CHANGELOG.md
• fd83ac2 by @rafamizes: doc: documentation improvement...

Released by Rultor 2.0-SNAPSHOT, see build log