Updated to coarse control of portamento time

Changed from fine to coarse control of portamento time based on feedback at #49 (comment)

Author: deckerego

adafruit_midi/control_change_values.py

@@ -44,7 +44,7 @@
 VOLUME = 7
 PAN = 10
 EXPRESSION = 11
-PORTAMENTO_TIME = 37
+PORTAMENTO_TIME = 5
 SUSTAIN_PEDAL = 64
 PORTAMENTO = 65
 FILTER_RESONANCE = 71