Re-enable solidity coverage

- Upgrade version
- Re-enable in travis.yml
- Move mocks to contracts folder for instrumentation

contracts/mocks/SafeMathMock.sol

@@ -0,0 +1,21 @@
+pragma solidity ^0.4.18;
+
+
+import '../math/SafeMath.sol';
+
+
+contract SafeMathMock {
+  uint256 public result;
+
+  function multiply(uint256 a, uint256 b) public {
+    result = SafeMath.mul(a, b);
+  }
+
+  function subtract(uint256 a, uint256 b) public {
+    result = SafeMath.sub(a, b);
+  }
+
+  function add(uint256 a, uint256 b) public {
+    result = SafeMath.add(a, b);
+  }
+}