Test exit status of `spring rspec`

If the specs fail, then the command should have a non-zero exit status.
Correct exit status for `rspec` and `cucumber` commands

Tests from the previous commit now pass. For reference, see the documentation of
RSpec::Core::Runner#run and the code of the cucumber executable.

We probably should put this logic into the fork block of Application#serve. But
unfortunately the different return values for Command#call makes this
impossible.
Comment unusual usage of return value

Exiting with an error when a function returns true is counterintuitive. But in
fact Cucumber's `Main::execute` returns false if everything ran cleanly and true
otherwise. So it actually looks more like `failed?`. For reference see
bin/cucumber:14.
Merge pull request #107 from geigerzaehler/rspec_exit

Correct exit status for `rspec` and `cucumber` commands