accept dot position on same line for trailing style

Hi,

I ran into trouble today when I tried to use the trailing style with the DotPosition cop. The cop only accepts notation where the dot and the method name are on different lines. Even with single line expressions like 'object.method'. 
To fix it I added a check to see if multiple lines are involved. The test I added works just fine.

However I know next to nothing about the ast and parser gems and it is thus very likely a better solution exists.
Anyway I hope this helps resolving the mentioned issue.

Cheers,
Hagen