[pod] Use `swift_versions` in spec and remove .swift-version

.swift-version

@@ -1 +0,0 @@
-4.2

SwiftBase32.podspec

@@ -12,4 +12,5 @@ Pod::Spec.new do |s|
   s.osx.deployment_target     = '10.9'
   s.tvos.deployment_target    = '9.0'
   s.watchos.deployment_target = '2.0'
+  s.swift_versions            = ['4.2', '5.0', '5.1']
 end