Fully convert to rspec3's expect() syntax

Mostly performed using the yujinakayama/transpec tool, with minor manual tweaking for readability.

This silences the deprecation warnings rspec spits out after a fresh `bundle`.
Plopped a pry in here looking into the rbx failure.

On ruby 2.1.x:

~~~
[1] pry(#<RSpec::ExampleGroups::LookupByCachingLRU>)> [ merged.class, merged.keys ]
=> [LookupBy::Caching::LRU, [1, 2, 3]]
~~~

On rbx 2.2.9:

~~~
[1] pry(#<RSpec::ExampleGroups::LookupByCachingLRU>)> [ merged.class, merged.keys ]
=> [Hash, [1, 3]]
~~~

Yet this works fine on both:

~~~
Wtf = Class.new(Hash)
wtf = Wtf.new
wtf[1] = 'one'
wtf.merge(1 => 'change', 2 => 'new').class #=> Wtf
~~~