iOS 9 and OSX 10.11 require TLSv1.2 SSL for all hosts you plan to request data from unless you specify exception domains in your app’s Info.plist file.
The syntax for the Info.plist configuration looks like this:
<key>NSAppTransportSecurity</key>
<dict>
<key>NSExceptionDomains</key>
<dict>
<key>yourserver.com</key>
<dict>
<!--Include to allow subdomains-->
<key>NSIncludesSubdomains</key>
<true/>
<!--Include to allow insecure HTTP requests-->
<key>NSExceptionAllowsInsecureHTTPLoads</key>
<true/>
<!--Include to specify minimum TLS version-->
<key>NSExceptionMinimumTLSVersion</key>
<string>TLSv1.1</string>
</dict>
</dict>
</dict>
If your application (a third-party web browser, for instance) needs to connect to arbitrary hosts, you can configure it like this:
<key>NSAppTransportSecurity</key>
<dict>
<!--Connect to anything (this is probably BAD)-->
<key>NSAllowsArbitraryLoads</key>
<true/>
</dict>
If you’re having to do this, it’s probably best to update your servers to use TLSv1.2 and SSL, if they’re not already doing so. This should be considered a temporary workaround.
As of today, the prerelease documentation makes no mention of any of these configuration options in any specific way. Once it does, I’ll update the answer to link to the relevant documentation.