Two gemfiles, each for one rspec major version.

* Gemfiles are only for development
* we still support legacy projects with RSpec2 because we can do that
  without much effort and know people in such position
Loosen rspec version requirement.

* I don't see reason why we should enforce at least 2.6 version,
  it's quite old already but nothing is blocking us here
Don't store locks.

* I want bundler to fetch newest release, especially on CI
* when I want to lock to specific version, I can always do that manually
  or introducing another, more strict Gemfile
More sanity in tests.

* no need to run rspec from testunit, just see if matchers do what we
  expect
* who needs rails app here, anyway?