iOS SWIFT_VERSION missing issue

[xiyulangzi]

when build iOS on Catalina, it will fail with error message that SWIFT_VERSION is missing,

it can be fixed by add below lines in flutter_vpn.podspec

  s.ios.deployment_target = '8.0'

  #added to make it working for iOS
  # Flutter.framework does not contain a i386 slice. Only x86_64 simulators are supported.
  s.pod_target_xcconfig = { 'DEFINES_MODULE' => 'YES', 'VALID_ARCHS[sdk=iphonesimulator*]' => 'x86_64' }
  s.swift_version = [ '4.0', '4.1', '4.2', '5.0', '5.1' ]
  

so please fix this in next release.

[ctrysbita]

Can you make a pull request to fix it? I cannot make sure it works since I don't have a iOS device and macOS environment to test it. Thanks.