Yosemite: Codesign fix

Updates Sparkle to 1.8.0 and removes the parts that overrides RubyMotion's codesign method.

Fixes https://github.com/webcracy/motion-sparkle/issues/9#issuecomment-58343882

If you are running RubyMotion 2.37 or earlier you'll have to apply a patch to the build process, as [explained by @alloy](https://github.com/webcracy/motion-sparkle/issues/9#issuecomment-59768849). The patch is [available here](https://gist.github.com/alloy/ee1fe70379dbbbb0d827). I'm assuming the fix will be released in the next version of RM.

You should see the framework being signed in the build process:
```
Build ./build/MacOSX-10.9-Release
(...)
Codesign ./build/MacOSX-10.9-Release/Screenmailer.app/Contents/Frameworks/Sparkle.framework/Versions/A
Codesign ./build/MacOSX-10.9-Release/Screenmailer.app
```
Thanks a lot of the help, @alloy.