Background gradients will be ignored on jQM 1.4 and over. Fixes #149.

Hi~!
I'll a new PR for #149 issue instead of wrong PR https://github.com/jquery/web-jquery-mobile-theme-roller/pull/155.

As discussed, on jQM 1.4 and over, background gradients will be ignored and gradient controls(gradient slider bar, "+" button, and input box for START color and END color) is removed on left Swatch panel/menu.

If there are any problems or mistakes on that PR, please let me know.
Thanks in advance.