Re-enable solidity coverage

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

contracts/mocks/FinalizableCrowdsaleImpl.sol

@@ -0,0 +1,20 @@
+pragma solidity ^0.4.18;
+
+
+import '../crowdsale/FinalizableCrowdsale.sol';
+
+
+contract FinalizableCrowdsaleImpl is FinalizableCrowdsale {
+
+  function FinalizableCrowdsaleImpl (
+    uint256 _startTime,
+    uint256 _endTime,
+    uint256 _rate,
+    address _wallet
+  ) public
+    Crowdsale(_startTime, _endTime, _rate, _wallet)
+    FinalizableCrowdsale()
+  {
+  }
+
+}