Bump version from 2.4.0 to 3.0.0: 2.4.0 introduced a breaking change!

The change from e61570c (see
https://github.com/wordpress-mobile/WordPressAuthenticator-iOS/pull/673/files#diff-6af3d0411d19c7a69f45065597580a918c1566418bd09ac867b56ba05cd9dbbeL63-R71
on GitHub) changed the `WordPressAuthenticatorDelegate` public
interface in a breaking way.

The breaking result can be seen in the following WordPress iOS build,
where we upgraded WordPressAuthenticator following Semantic Versioning
and experienced a build failure:

```
WordPress/Classes/ViewRelated/NUX/WordPressAuthenticationManager.swift:228:1: type 'WordPressAuthenticationManager' does not conform to protocol 'WordPressAuthenticatorDelegate'
```

https://buildkite.com/automattic/wordpress-ios/builds/9808#01830b2b-76f3-4fe1-b385-52e09f417c3c/1745-8411

Author: mokagio

WordPressAuthenticator.podspec

@@ -2,7 +2,7 @@
 
 Pod::Spec.new do |s|
   s.name          = 'WordPressAuthenticator'
-  s.version       = '2.4.0'
+  s.version       = '3.0.0'
 
   s.summary       = 'WordPressAuthenticator implements an easy and elegant way to authenticate your WordPress Apps.'
   s.description   = <<-DESC