Bump valid_email2 from 3.6.1 to 4.0.0

Bumps [valid_email2](https://github.com/micke/valid_email2) from 3.6.1 to 4.0.0.
- [Release notes](https://github.com/micke/valid_email2/releases)
- [Changelog](https://github.com/micke/valid_email2/blob/master/CHANGELOG.md)
- [Commits](micke/valid_email2@v3.6.1...v4.0.0)

Gemfile.lock

@@ -610,7 +610,7 @@ GEM
     unicorn (6.0.0)
       kgio (~> 2.6)
       raindrops (~> 0.7)
-    valid_email2 (3.6.1)
+    valid_email2 (4.0.0)
       activemodel (>= 3.2)
       mail (~> 2.5)
     vcr (6.0.0)