* Fix AtomicInt usages of deprecated atomic APIs

As of iOS 10.0 and MacOS 10.12 the old `OSMemoryBarrier`, `OSAtomicCompareAndSwap64Barrier`, `OSAtomicIncrement64Barrier`, and `OSAtomicDecrement64Barrier, APIs are deprecated. The warnings suggested new APIs are C++ and are not accessible from Swift.

`OSMemoryBarrier` can be directly replaced with `atomic_thread_fence(memory_order_seq_cst)` which is accessible from Swift.

The rest of the suggested C++ APIs `atomic_fetch_add`, `atomic_fetch_sub`, and `atomic_compare_exchange_strong` are not accessible from Swift. This change creates a Objective-C wrapper for these C++ APIs, so Swift code can access them.

* Update TravisCI to use Xcode 10.1

* Allow task execution to throw errors

Adds support to allow task execution to throw errors. The error is then reported back via the `SequenceExecutionHandle.await` method when results are being retrieved.

* Ignore SPM generated scheme

* Xcode project changes

Similar to `DispatchSemaphore`, `AutoReleasingSemaphore` is a synchronization mechanism that ensures only a set number of threads can concurrently access a protected resource. Unlike `DispatchSemaphore`, `AutoReleasingSemaphore` auto-releases all blocked threads when the semaphore itself is deallocated.

This fixes the issue where if a task throws an error during execution, the concurrency limiting semaphore is never signaled. This can hault the entire sequence since no new tasks can continue execution.

* Update ConcurrentSequenceExecutor to use AutoReleasingSemaphore

Fixes #29

* Remove debugging code

Fix Carthage Build