Tweak bishop pair and knight weight

A combo of two patches that failed SPRT with score
higher than 50% but togheter they succeed:

SPRT at 60+0.05
LLR: 2.95 (-2.94,2.94) [0.00,6.00]
Total: 7312 W: 1276 L: 1139 D: 4897

bench: 8029334
Merge pull request #21 from mcostalba/master

Tweak bishop pair and knight weight