Re-enable solidity coverage

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

contracts/mocks/CappedCrowdsaleImpl.sol

@@ -0,0 +1,21 @@
+pragma solidity ^0.4.18;
+
+
+import '../crowdsale/CappedCrowdsale.sol';
+
+
+contract CappedCrowdsaleImpl is CappedCrowdsale {
+
+  function CappedCrowdsaleImpl (
+    uint256 _startTime,
+    uint256 _endTime,
+    uint256 _rate,
+    address _wallet,
+    uint256 _cap
+  ) public
+    Crowdsale(_startTime, _endTime, _rate, _wallet)
+    CappedCrowdsale(_cap)
+  {
+  }
+
+}