Upgrade to apn version 1.6.0.

Upgraded to apn version 1.6.0, which fixes several issues with socket.

+1.6.0:
 +
 +* Fixes #156: Retire a socket from the pool immediately instead of waiting for a "close" event.
 +* Fixes #165: Introduced a connection retry limit to emit errors when fatal connection errors occur.
 +* Fixes #152: Failure to attach an error handler would prevent exponential backoff from working.
 +* Improved error handling for EPIPE write events to maximise chances of reading error info from Apple.
 +* Fixed #181: Initial support for larger payloads for iOS 8 testing.
 +* Fixed #187: Correctly trim notifications with escaped characters.
 +* Merged #190: Always emit a feedback event when batch mode is enabled, even if there are no tokens. 
 +* Merged #189: Added support for iOS 8 Notification actions.
 +* Display a warning when null parameters are passed during connection creation.
 +* Fixed #171: Removed references to the unknown notification identifier.

Signed-off-by: Aris Kemper <aris.github@gmail.com>