Fully convert to rspec3's expect() syntax

Mostly performed using the yujinakayama/transpec tool, with minor manual
tweaking for readability.
Support `fit` by defaulting to filter on `:focus`
LRU#merge! should return self

The spec around `LRU#merge` was also simply wrong -- with an LRU size of
2, once we merge 2 keys in, one has to be thrown away.

Rubinius was actually *right*; I needed to reimplement `merge` in the
obvious manner to fix MRI.
Merge pull request #16 from pd/expect

Fully convert to rspec3's expect() syntax