Update version in README

[ci skip]

Author: mcmire

README.md

@@ -10,7 +10,7 @@
 SuperDiff is a gem that hooks into RSpec to intelligently display the
 differences between two data structures of any type.
 
-📢 **[See what's changed in the latest version (0.3.0)][changelog].**
+📢 **[See what's changed in the latest version (0.4.0)][changelog].**
 
 [changelog]: CHANGELOG.md