Merge pull request #335 from lurch/patch-1

Fix Travis-build Recipe

docs/recipes.rst

@@ -328,6 +328,7 @@ tests are passing, a red light means the build is broken::
 
     from travispy import TravisPy
     from gpiozero import LED
+    from gpiozero.tools import negated
     from time import sleep
     from signal import pause
 
@@ -338,14 +339,10 @@ tests are passing, a red light means the build is broken::
             yield r.last_build_state == 'passed'
             sleep(delay) # Sleep an hour before hitting travis again
 
-	def invert(values):
-		for value in values:
-			yield not value
-
     red = LED(12)
     green = LED(16)
 
-	red.source = invert(green.values)
+    red.source = negated(green.values)
     green.source = build_passed()
     pause()