Fix for system colors on OSX

Untested since I can't run bundle `exec rake spec platform=osx` properly in RM 2.37 on Yosemite. Errors out with:

```
2014-10-30 12:24:27.960 SugarCube[15846:2228237] Failed to connect (colorGridView) outlet from (NSApplication) to (NSColorPickerGridView): missing setter or instance variable
2014-10-30 12:24:27.960 SugarCube[15846:2228237] Failed to connect (view) outlet from (NSApplication) to (NSColorPickerGridView): missing setter or instance variable
2014-10-30 12:24:28.346 SugarCube[15846:2228237] *nil description*
```

We'll see what travis has to say about it!
this method is available in 10.0 and later - why the check?