Fix BoldSystemFontOfSize obsolete warning

The FormsGallery had a bunch of warnings. I fixed them.

![fix warning](https://cloud.githubusercontent.com/assets/1173057/3710736/cc2f2cfa-14a6-11e4-9a98-bcd1c03de2ec.png)

>  Warning CS0618: `Xamarin.Forms.Font.BoldSystemFontOfSize(double)' is obsolete: `BoldSystemFontOfSize is obsolete, please use SystemFontOfSize (double, FontAttributes)' (CS0618) (FormsGallery)