Bugfix: correcting bitshift order to match ws2811 spec

[phleech]

Whilst testing this library against a string of 50 WS2811 LED's I noticed that Colour(r, g, b) seemed to be mixing up the red and green channels.

Upon investigation it appears that this is a result of the bit shifting being incorrect.

Excerpt from the WS2811 spec:

Composition of 24bit Data
R7 R6 R5 R4 R3 R2 R1 R0 G7 G6 G5 G4 G3 G2 G1 G0 B7 B6 B5 B4 B3 B2 B1 B0

This is a breaking change. It now works correctly for my string of LED's but could potentially break for others (especially if they've added work arounds in their code for the issue).