Tune PawnsFileSpan

Passed the following SPRT tests:

STC:
LLR: 2.96 (-2.94,2.94) [-1.50,4.50]
Total: 24428 W: 5056 L: 4880 D: 14492

LTC:
LLR: 2.96 (-2.94,2.94) [0.00,4.00]
Total: 26590 W: 4715 L: 4472 D: 17403

Bench: 6615949

Resolves #78

src/pawns.cpp

@@ -58,7 +58,7 @@ namespace {
     S(20,20), S(40,40), S(0, 0), S(0, 0) };
 
   // Bonus for file distance of the two outermost pawns
-  const Score PawnsFileSpan = S(0, 15);
+  const Score PawnsFileSpan = S(0, 8);
 
   // Unsupported pawn penalty
   const Score UnsupportedPawnPenalty = S(20, 10);