Replace deprecated methods

[djih]

Removing 2 deprecated methods (#67) since Xcode is now enforcing that you don't use these methods, will likely block iOS 11 updates.

CFURLCreateStringByAddingPercentEscapes is simple to replace with stringByAddingPercentEncodingWithAllowedCharacters

NSURLConnection is more involved to replace with NSURLSession. Since we aren't altering any of the connection configuration settings, we can just use [NSURLSession sharedSession]. It's a little trickier with SSL pinning. I added an AMPURLSession class that extends ISPPinnedNSURLSessionDelegate and wraps NSURLSession. I'm having to create a new NSURLSession each time you try to upload events, similar to how the previous AMPURLConnection creates a new connection with each request. Although I think you're supposed to try to re-use NSURLSessions, but I wasn't having much luck with a static instance in AMPURLSession.

This will bump the min iOS version to 9. This also verifies that the unit tests pass in iOS 11 (on iPhone X emulator). Will need to do some more additional testing on the demo app for the SSL pinning

[curtisliu]

What was the issue you were running into with a static instance in AMPURLSession? Was it not possible to mirror the sharedSession call of NSURLSession?

[djih]

Switched to using a static instance of the AMPURLSession delegate that maintains a persistent sharedSession that it can pass function calls to. Verified that the pinning works in our demo app running on an iphone X emulator with iOS 11. Also bumped the iOS target in podfile to 9.0 (no more 6.0). PTAL @curtisliu

[curtisliu]

LGTM