fixed MPRIS Previous method according to the specs

Using conditional previous in the MPRIS interface introduced some bugs.
It was not possible to go to the previous track via plasma's MPRIS client.

src/mpdevil.py

@@ -362,7 +362,7 @@ class MPRISInterface:  # TODO emit Seeked if needed
 		self._client.next()
 
 	def Previous(self):
-		self._client.conditional_previous()
+		self._client.previous()
 
 	def Pause(self):
 		self._client.pause(1)