Math.average (#1170)

* Added Math.average

* Removed assertion.

contracts/mocks/MathMock.sol

@@ -12,4 +12,8 @@ contract MathMock {
   function min(uint256 _a, uint256 _b) public pure returns (uint256) {
     return Math.min(_a, _b);
   }
+
+  function average(uint256 _a, uint256 _b) public pure returns (uint256) {
+    return Math.average(_a, _b);
+  }
 }