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.