Re-enable solidity coverage

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

contracts/mocks/SecureTargetBounty.sol

@@ -0,0 +1,17 @@
+pragma solidity ^0.4.18;
+
+
+import {Bounty, Target} from "../../contracts/Bounty.sol";
+
+
+contract SecureTargetMock is Target {
+  function checkInvariant() public returns(bool) {
+    return true;
+  }
+}
+
+contract SecureTargetBounty is Bounty {
+  function deployContract() internal returns (address) {
+    return new SecureTargetMock();
+  }
+}