Support for SLIM 4

What's Changed

• Make _register_engine and _register_engine3 private by @hkdnet in #154
• [CI] Test against Ruby 2.5 by @nicolasleger in #151
• Relax dependency for slim v4.0.0 by @sue445 in #155
• Improve travis build by @tagliala in #157

New Contributors

• @hkdnet made their first contribution in #154
• @sue445 made their first contribution in #155

Full Changelog: v3.1.3...v3.2.0