Add pods to default podspec for network stubbing

- OHHTTPStubs is a fantastic library for quickly and easily stubbing
 network requests
- XCTest+OHHTTPStubSuiteCleanUp ensures that all stubs are reset after
 each test. This ensures that tests aren't dependent on interacting
 with each other. Just including this pod enables this feature. There
 is no need to import the pod anywhere.