Add 120 x 120 size to default options

Mathias Bynens' [touch icons post]( http://mathiasbynens.be/notes/touch-icons#sizes) was updated a while ago with the following note:

> Update (June 2013): As of iOS 7, the recommended touch icon size for Retina-display iPhones went up from 114 × 114 pixels to 120 × 120 pixels. This post has been updated to reflect that.

Looks to be a relatively straightforward change so I went straight in for a pull request. The recommended code snippet keeps the 114 x 114 size alongside the new 120 x 120 size, so I've just added the new size, bumped test counts in the gemspec and amended the README.