Update podspace version.

Mobile-Buy-SDK.podspec

 Pod::Spec.new do |s|
   s.name         = 'Mobile-Buy-SDK'
-  s.version      = '1.2.6'
+  s.version      = '2.0.0'
   s.summary      = 'Sell with Shopify in iOS apps'
   s.description  = 'Shopify’s Mobile Buy SDK makes it simple to sell physical products inside your mobile app. With a few lines of code, you can connect your app with the Shopify platform and let your users buy your products using Apple Pay or their credit card.'
   s.homepage     = 'https://developers.shopify.com/mobile-buy-sdk'