Bumping version to 1.2.0

Alamofire.podspec

 Pod::Spec.new do |s|
   s.name = 'Alamofire'
-  s.version = '1.1.5'
+  s.version = '1.2.0'
   s.license = 'MIT'
   s.summary = 'Elegant HTTP Networking in Swift'
   s.homepage = 'https://github.com/Alamofire/Alamofire'