update podspec: added swift version 4.0

Author: michaelhenry

MHFacebookImageViewer.podspec

@@ -12,7 +12,7 @@ Pod::Spec.new do |s|
   s.requires_arc = true
   s.dependency 'SDWebImage'
   s.ios.deployment_target  = '10.0'
-  s.swift_versions = ['4.2','5.0']
+  s.swift_versions = ['4.0','4.2','5.0']
   s.license = {
     :type => 'MIT',
     :text => <<-LICENSE