Ring modulo crash

[rowlap]

Although the ring() and single() library functions allow any integer value for ring_index, the code crashes when passed certain values due to modulo 7, which should be modulo 6 to correspond with the size of the lists in _legs.

Steps to reproduce, creating a wormhole-in-space effect:

$ cat piglow-ring-modulo-bug.py
import piglow

for x in range(20):
    piglow.ring(x, 100)
    piglow.ring(x+1, 0)
    piglow.show()

$ python3 piglow-ring-modulo-bug.py
Traceback (most recent call last):
  File "/home/pi/piglow-ring-modulo-bug.py", line 5, in <module>
    piglow.ring(x+1, 0)
  File "/home/pi/piglow-pristine/piglow/library/piglow.py", line 181, in ring
    _set([_legs[0][ring_index], _legs[1][ring_index], _legs[2][ring_index]], value)
IndexError: list index out of range

[rowlap]

Resolved in PR #21