Why appium doesn't work with ruby?

Help Please.

I use:win 7, ruby 2.0.0, appium 1.3.7.0

I test android app.

In test suite, in my steps I define:

Appium.back

I try tap on hardware(native) button e.g. back, home.

but if I run test, appears next error :

NoMethodError: undefined method 'back' for Appium:Module