Add swift version to podspec and remove deprecated .swift-version file

Author: Tyler Neveldine

.swift-version

@@ -4,6 +4,7 @@ s.version               = '0.1.3'
 s.license               = { :type => 'MIT', :file => 'LICENSE' }
 s.platform              = :ios, "8.0"
 s.ios.deployment_target = '8.0'
+s.swift_version         = '5.0'
 s.authors               = { 'Shady Elyaski' => 'shady@elyaski.com' }
 s.screenshots           = 'https://raw.github.com/DynamicSignal/ios-twitter-text-counter/master/Example/Assets/sample.gif'
 s.homepage              = 'https://github.com/DynamicSignal/ios-twitter-text-counter'