Merge pull request #5348 from ethereum/boostRationalNegativeDenominatorFix

Fix negative denominator in ``boost::rational`` during exponentiation.

test/libsolidity/syntaxTests/types/rational_negative_numerator_negative_exp.sol

@@ -0,0 +1,5 @@
+contract C {
+    function f() public pure returns (int) {
+        return (-1 / 2) ** -1;
+    }
+}