Update version in sample configurations of the README to 4.2

This makes sure that users who are new to the project start using the new version.
Otherwise, if they use Swift 4.2, they will not be able to successfully build
their project.

Author: wtimme

README.md

@@ -477,7 +477,7 @@ The preferred way is to use _git_ directly or _Carthage_.
 Installation via Carthage is easy enough:
 
 ```ruby
-github "p2/OAuth2" ~> 3.0
+github "p2/OAuth2" ~> 4.2
 ```
 
 #### git
@@ -514,7 +514,7 @@ If you're unfamiliar with CocoaPods, read [using CocoaPods](http://guides.cocoap
 platform :ios, '8.0'          # or platform :osx, '10.9'
 use_frameworks!
 target `YourApp` do
-  pod 'p2.OAuth2', '~> 3.0'
+  pod 'p2.OAuth2', '~> 4.2'
 end
 ```