Fix RSpec 3 compatibility (partially)

I'm finally able to use Webmachine for something again, and first thing I notice is the failing build ;) Most of the remaining spec failures look like they're related to RSpec 3, but quite some stuff happened on master since the build broke, so I'd like someone to doublecheck before I waste time. If you agree they look like RSpec 3 fallout, I'll happily fix them.

- `let` is deprecated, require rspec-its instead
- `be_false/be_true` tests for `false?/true?` now, switch to `be(false|true)`