Fix slight wobble in temperature caused by float.

The transition variable 'adjustment_alpha' is a float. Redshift will wobble between 6499 and 6500K as the float runs out of precision. Using a double seems to solve the issue.
(Thanks to Sean Hildebrand)

src/redshift.c

@@ -1168,7 +1168,7 @@ main(int argc, char *argv[])
 		/* Amount of adjustment to apply. At zero the color
 		   temperature will be exactly as calculated, and at one it
 		   will be exactly 6500K. */
-		float adjustment_alpha = 0.0;
+		double adjustment_alpha = 0.0;
 
 #if defined(HAVE_SIGNAL_H) && !defined(__WIN32__)
 		struct sigaction sigact;