Re-enable solidity coverage

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

contracts/mocks/StandardTokenMock.sol

@@ -0,0 +1,15 @@
+pragma solidity ^0.4.18;
+
+
+import '../token/StandardToken.sol';
+
+
+// mock class using StandardToken
+contract StandardTokenMock is StandardToken {
+
+  function StandardTokenMock(address initialAccount, uint256 initialBalance) public {
+    balances[initialAccount] = initialBalance;
+    totalSupply = initialBalance;
+  }
+
+}