Release iOS SDK v12.1.3

[danj-stripe]

• Update README
• Update CHANGELOG with any new features or breaking changes
• Update MIGRATING (if necessary)
• Update STPSDKVersion in STPAPIClient.h
• Update version in Info.plist (make sure to not change the "InfoDictionary" field)
• Update VERSION
• Update Stripe.podspec (make sure there aren't any new directories added, and if so, add them to the "files" section
• Update documentation: ./ci_scripts/build_documentation.sh
• Make sure pod lib lint succeeds without warnings or errors.
• Make sure ./ci_scripts/export_builds.sh succeeds without warnings or errors

[danj-stripe]

./ci_scripts/export_builds.sh fails on my machine with Xcode 9.2. Both 9.1 and 9.3 work.

While building module 'PassKit' imported from /Users/danj/stripe/stripe-ios/Stripe/PublicHeaders/STPBlocks.h:10:
In file included from <module-includes>:1:
In file included from /Applications/Xcode 9.2.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.2.sdk/System/Library/Frameworks/PassKit.framework/Headers/PassKit.h:38:
/Applications/Xcode 9.2.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.2.sdk/System/Library/Frameworks/PassKit.framework/Headers/PKPaymentAuthorizationViewController.h:148:99: error: expected a type
+ (BOOL)canMakePaymentsUsingNetworks:(NSArray<PKPaymentNetwork> *)supportedNetworks capabilities:(PKMerchantCapability)capabilties API_AVAILABLE(ios(9.0));
                                                                                                  ^
/Applications/Xcode 9.2.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.2.sdk/System/Library/Frameworks/PassKit.framework/Headers/PKPaymentAuthorizationViewController.h:156:50: error: expected a type
- (nullable instancetype)initWithPaymentRequest:(PKPaymentRequest *)request NS_DESIGNATED_INITIALIZER;
                                                 ^
While building module 'PassKit' imported from /Users/danj/stripe/stripe-ios/Stripe/PublicHeaders/STPBlocks.h:10:
In file included from <module-includes>:1:
In file included from /Applications/Xcode 9.2.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.2.sdk/System/Library/Frameworks/PassKit.framework/Headers/PassKit.h:41:
/Applications/Xcode 9.2.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.2.sdk/System/Library/Frameworks/PassKit.framework/Headers/PKPaymentAuthorizationController.h:125:99: error: expected a type
+ (BOOL)canMakePaymentsUsingNetworks:(NSArray<PKPaymentNetwork> *)supportedNetworks capabilities:(PKMerchantCapability)capabilties;
                                                                                                  ^
/Applications/Xcode 9.2.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS11.2.sdk/System/Library/Frameworks/PassKit.framework/Headers/PKPaymentAuthorizationController.h:131:41: error: expected a type
- (instancetype)initWithPaymentRequest:(PKPaymentRequest *)request NS_DESIGNATED_INITIALIZER;
                                        ^
4 errors generated.
In file included from /Users/danj/stripe/stripe-ios/Stripe/STPSourcePoller.m:9:
In file included from /Users/danj/stripe/stripe-ios/Stripe/STPSourcePoller.h:10:
/Users/danj/stripe/stripe-ios/Stripe/PublicHeaders/STPBlocks.h:10:9: fatal error: could not build module 'PassKit'
#import <PassKit/PassKit.h>
 ~~~~~~~^
5 errors generated.

[danj-stripe]

Must be something wrong with that copy of Xcode. A fresh download works, and it works fine on joeydong-stripe's machine.

[danj-stripe]

Closed in favor of #933, due to #931 (a breaking change)