[v13.8.0] Update podspec

Author: younata

Nimble.podspec

@@ -1,6 +1,6 @@
 Pod::Spec.new do |s|
   s.name         = "Nimble"
-  s.version      = "13.7.1"
+  s.version      = "13.8.0"
   s.summary      = "A Matcher Framework for Swift and Objective-C"
   s.description  = <<-DESC
                    Use Nimble to express the expected outcomes of Swift or Objective-C expressions. Inspired by Cedar.