Finalizing #1412 fix for __IPHONE_OS_VERSION_MIN_REQUIRED

#1412 only fixed the replacement of `__IPHONE_OS_VERSION_MIN_REQUIRED` to `__IPHONE_OS_VERSION_MAX_REQURED` in `AFURLSessionManager.h/m`.

This fixes this for `AFHTTPSessionManager.h/m` too, so that we can use AFN2 with projects that still want to support iOS<7 (and have a Deployment Target < iOS7 but tests for `NSURLSession.class` existence at runtime to use it conditionally)

_(Note: This additionally allows the code to compile with Xcode4/SDK6, which is typically a must-have for all projects around here that use Travis-CI (as [Travis-CI's migration to Xcode5 is still pending](https://github.com/travis-ci/travis-ci/issues/1422#issuecomment-26151024)) because those Travis builds will keep failing until this fix is merged)_