Update PayPal mSDK to 2.5.0

Require MobileCoreServices for PayPal subspec
Use new PayPal "willLogin" delegate method with completion callback

Adapt BTPaymentProvider and BTDropInViewController to new PayPal payPalProfileSharingViewController:userWillLogInWithAuthorization:completionBlock: delegate method
Add modal presentation option switch to dev app
Add CardIO*.h headers to PayPal subspec public headers
Drop In - Dismiss modal Drop In before presenting PayPal in app login
Drop In - Dismiss inner Drop-In before app switching
Add additional system frameworks to paypal target: CoreMedia and AudioToolbox
Merge pull request #81 from braintree/feature/fix-double-dismiss-with-paypal-msdk-2.5.0

Upgrade to PayPal mSDK 2.5.0, fix double dismiss bug, hide behind PayPal spinner, add card.io headers
Drop In - Fix example summary banner text
Use both BTPaymentProvider and client token verison of supportedNetworks for determining whether Apple Pay will be possible
Merge remote-tracking branch 'public/master'
Update CHANGELOG for 3.4.0, remove release candidates from CHANGELOG
Remove PRE-RELEASE APPLE PAY warnings
Bump pod version to 3.4.0-rc4
Include Card.io headers in Xcode project
Add client analytics instrumentation to Apple Pay integrations
Check nil before calling block in Apple Pay tokenization
Add card.io demonstration in demo app
Allow nil placeholders in form fields

* Fixes exception thrown by NSAttributedString
Remove deprecated test around tokenizeApplePayPayment: