Bump version to 2.4.0.

PromisesObjC.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name        = 'PromisesObjC'
-  s.version     = '2.3.1'
+  s.version     = '2.4.0'
   s.authors     = 'Google Inc.'
   s.license     = { :type => 'Apache-2.0', :file => 'LICENSE' }
   s.homepage    = 'https://github.com/google/promises'

PromisesSwift.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name        = 'PromisesSwift'
-  s.version     = '2.3.1'
+  s.version     = '2.4.0'
   s.authors     = 'Google Inc.'
   s.license     = { :type => 'Apache-2.0', :file => 'LICENSE' }
   s.homepage    = 'https://github.com/google/promises'

g3doc/index.md

@@ -414,7 +414,7 @@ In your `Package.swift` file, add `Promises` dependency to corresponding targets
 let package = Package(
   // ...
   dependencies: [
-    .package(url: "https://github.com/google/promises.git", from: "2.3.1"),
+    .package(url: "https://github.com/google/promises.git", from: "2.4.0"),
   ],
   // ...
 )