update kotlin and gradle version

bump version

Bump Kotlin gradle plugin version (#17)

Fixed wrong variable name in README (#13)

Bump Kotlin version to 1.2.50 (#9)

Update CHANGELOG.md

Update flutter_string_encryption.podspec

Update pubspec.yaml

update CHANGELOG and version

update project file

Make PlatformStringCryptor easier to extend

Remove useless line in build.gradle - Fix #6

Specify sdk

Fix Dart 2 issues

Update README.md