Specify deployment targets in pbxproj file

[spinach]

This change is needed for Carthage to operate well, because if we don't specify them, then carthage will assume a minimum deployment target of 12.1 for tvOS which is not what we want.

I followed the same versions listed in the podspec:

  s.ios.deployment_target = '8.0'
  s.osx.deployment_target = '10.9'
  s.tvos.deployment_target = '9.0'
  s.watchos.deployment_target = '2.0'