ltxmdf.cls depends on DejaVuSans, fails to compile if not available

[mithodin]

I tried to compile mdframed on a mac where I can't install any packages system-wide. Therefore, I can't install DejaVuSans, but ltxmdf.cls seems to need it (though commenting out line 60 seems to result in a working installation). Please remove that dependency if it is not absolutely necessary or give a possibility to switch it off.