[modules/kernel] Use Python standard module "platform"

Instead of executing an external call to "uname", use the standard
Python module "platform" to retrieve information about the kernel used.

Positive side-effect: This is portable, if i3 ever exists on Windows :P

bumblebee/modules/kernel.py

@@ -2,6 +2,8 @@
 
 """Shows Linux kernel version information"""
 
+import platform
+
 import bumblebee.input
 import bumblebee.output
 import bumblebee.engine
@@ -11,7 +13,7 @@ class Module(bumblebee.engine.Module):
         super(Module, self).__init__(engine, config,
             bumblebee.output.Widget(full_text=self.output)
         )
-        self._release_name = bumblebee.util.execute("uname -r")[:-1]
+        self._release_name = platform.release()
 
     def output(self, widget):
         return self._release_name