Update invisible_captcha gem from 1.1.0 to 2.0.0

v2.0.0 is tested against Rails >= 5.2 and Ruby >= 2.5.

Backwards incompatible changes include:
- Drop official support for unmaintained Rails versions: 5.1, 5.0 and
4.2
- Drop official support for EOL Rubies: 2.4 and 2.3

See https://github.com/markets/invisible_captcha/blob/master/CHANGELOG.md#200

Changelog: other