Fix error "Unexpected duplicate tasks" when using Xcode new build system

Ensures images are not included in "Compile Sources" build phase by adding .swift extension to the podspec source_files:

s.ios.source_files = 'Sources/**/*.swift'

Otherwise images appear both in the Sources and in the resources bundle, which causes the new build system to complain about "Unexpected duplicate task":

After merging, please git tag 3.1.1 and then pod trunk push.

Thanks!