Small gas improvement in SafeMath.sol (#811)

contracts/math/SafeMath.sol

@@ -24,9 +24,9 @@ library SafeMath {
   */
   function div(uint256 a, uint256 b) internal pure returns (uint256) {
     // assert(b > 0); // Solidity automatically throws when dividing by 0
-    uint256 c = a / b;
+    // uint256 c = a / b;
     // assert(a == b * c + a % b); // There is no case in which this doesn't hold
-    return c;
+    return a / b;
   }
 
   /**