Re-enable solidity coverage

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

contracts/mocks/ReentrancyMock.sol

@@ -0,0 +1,44 @@
+pragma solidity ^0.4.18;
+
+import '../ReentrancyGuard.sol';
+import './ReentrancyAttack.sol';
+
+contract ReentrancyMock is ReentrancyGuard {
+
+  uint256 public counter;
+
+  function ReentrancyMock() public {
+    counter = 0;
+  }
+
+  function count() private {
+    counter += 1;
+  }
+
+  function countLocalRecursive(uint256 n) public nonReentrant {
+    if(n > 0) {
+      count();
+      countLocalRecursive(n - 1);
+    }
+  }
+
+  function countThisRecursive(uint256 n) public nonReentrant {
+    bytes4 func = bytes4(keccak256("countThisRecursive(uint256)"));
+    if(n > 0) {
+      count();
+      bool result = this.call(func, n - 1);
+      require(result == true);
+    }
+  }
+
+  function countAndCall(ReentrancyAttack attacker) public nonReentrant {
+    count();
+    bytes4 func = bytes4(keccak256("callback()"));
+    attacker.callSender(func);
+  }
+
+  function callback() external nonReentrant {
+    count();
+  }
+
+}