Upgrade to Unity 5.5

[sinbad]

I've just started trying out UnitySteer: thanks for creating it!

Unity 5.5 moves a few classes around and the auto-upgrade makes a bit of a mess of it, pre-pending the new namespace everywhere which will cause some merge issues down the line; the better solution is just to import the new namespaces, which is what this PR does.

Pretty simple but hope it's useful 😄

[ricardojmendez]

Thanks for the PR @sinbad. Give me few days to download Unity 5.5 and check it out (as well as what the issue is with Travis).