Allow rspec-core to be used without -expectations or -mocks.

At the moment if one wants to use RSpec's runner in standalone mode one
must first disable the default mocking/expectation adapters.

This patch silently sets up no mocking or expecting framework if the
default configuration is used but only 'rspec-core' is present in the
Gemfile.
Add cuke demonstrating that rspec-core can be used standalone.

Change require_expect_syntax_in_aruba_specs.rb to use the newer
`disable_monkey_patching!` option rather than configuring expectations
and mocks. Configuring those libs directly would fail when they
have been removed from the load path, and `disable_monkey_patching!`
provides the overall effect we want: monkey patching is removed
from all pieces of RSpec we use.
Changelog for #1615.