Upgrade to Bitters 2.0.4

The most important part of this Bitters release is the updated
dependency on Thor. We were seeing issues like:

```
Bundler could not find compatible versions for gem "thor":
  In snapshot (Gemfile.lock):
    thor (= 1.0.1)

  In Gemfile:
    suspenders was resolved to 1.53.0, which depends on
      bitters (~> 2.0) was resolved to 2.0.3, which depends on
        thor (~> 0.19)

    rails (~> 6.0.2, >= 6.0.2.1) was resolved to 6.0.2.1, which depends on
      railties (= 6.0.2.1) was resolved to 6.0.2.1, which depends on
        thor (>= 0.20.3, < 2.0)
```

Updating Bitters will fix that.

suspenders.gemspec

@@ -28,7 +28,7 @@ rush to build something amazing; don't use it if you like missing deadlines.
   s.test_files = `git ls-files -- {test,spec,features}/*`.split("\n")
   s.version = Suspenders::VERSION
 
-  s.add_dependency 'bitters', '~> 2.0'
+  s.add_dependency 'bitters', '>= 2.0.4'
   s.add_dependency 'rails', Suspenders::RAILS_VERSION
 
   s.add_development_dependency 'rspec', '~> 3.2'