Made some inherit-only contracts internal. (#1433)

* Made some inherit-only contracts internal.

* Added OwnableMock.

contracts/ownership/Ownable.sol

@@ -17,7 +17,7 @@ contract Ownable {
    * @dev The Ownable constructor sets the original `owner` of the contract to the sender
    * account.
    */
-  constructor() public {
+  constructor() internal {
     _owner = msg.sender;
     emit OwnershipTransferred(address(0), _owner);
   }

contracts/ownership/Secondary.sol

@@ -10,7 +10,7 @@ contract Secondary {
   /**
    * @dev Sets the primary account to the one that is creating the Secondary contract.
    */
-  constructor() public {
+  constructor() internal {
     _primary = msg.sender;
   }