Make ROS work with Ruby 2.4.x

[aetherknight]

[aetherknight]

Ruby 2.4.0-r1's ostruct.rb changes:

• modifiable is now modifiable?. The former aliases the latter, will be deprecated. It is now a private method.
• new_ostruct_member is now new_ostruct_member!. The former aliases to the latter, will be deprecated. It is now a private method.
• table is aliased to table!

There is one spec failing on 2.4.0, and it is due to the rename of new_ostruct_member. The rename means that ROS's implementation doesn't replace the original's so references to the method in OpenStruct do not call the ROS override.