Bump stripe from 18.2.0 to 18.4.0

Bumps [stripe](https://github.com/stripe/stripe-ruby) from 18.2.0 to 18.4.0.
- [Release notes](https://github.com/stripe/stripe-ruby/releases)
- [Changelog](https://github.com/stripe/stripe-ruby/blob/master/CHANGELOG.md)
- [Commits](https://github.com/stripe/stripe-ruby/compare/v18.2.0...v18.4.0)

---
updated-dependencies:
- dependency-name: stripe
  dependency-version: 18.4.0
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

Gemfile.lock

@@ -395,7 +395,7 @@ GEM
     sqlite3 (2.9.0)
       mini_portile2 (~> 2.8.0)
     stringio (3.1.7)
-    stripe (18.2.0)
+    stripe (18.4.0)
     superfish-rails (1.6.0.1)
     terminal-table (4.0.0)
       unicode-display_width (>= 1.1.1, < 4)