Simplify Keg class

[alyssais]

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The Keg class is pretty complex. It's scattered with repetitive method definitions that just forward to path. These are a couple of straightforward changes to make it a bit less difficult to navigate and, well… long.

As far as I can tell, the conditional aliasing of to_path is a remnant of Ruby 1.8.

I had a one-off test failure here as part of this test, but it seems unlikely to be related and I haven't been able to reproduce since…

[alyssais]

Huh, got the failure on Travis too.

[reitermarkus]

Nice refactoring. 👍