Re-enable solidity coverage

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

contracts/mocks/DayLimitMock.sol

@@ -0,0 +1,23 @@
+pragma solidity ^0.4.18;
+import "../../contracts/DayLimit.sol";
+
+contract DayLimitMock is DayLimit {
+  uint256 public totalSpending;
+
+  function DayLimitMock(uint256 _value) public DayLimit(_value) {
+    totalSpending = 0;
+  }
+
+  function attemptSpend(uint256 _value) external limitedDaily(_value) {
+    totalSpending += _value;
+  }
+
+  function setDailyLimit(uint256 _newLimit) external {
+    _setDailyLimit(_newLimit);
+  }
+
+  function resetSpentToday() external {
+    _resetSpentToday();
+  }
+
+}