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
Fix serialization of client environment.
Merge pull request #110 from oggy/fix-rails-env

Fix serialization of client environment.
make "spring test" support multiple arguments

"spring test" needs to accept multiple arguments if guard-minitest is
to ever have a chance of supporting spring.
make "spring test" run all unit tests when given no arguments

With no arguments, it now defaults to running all test-unit tests it
can find, i.e. files matching test/**/*_test.rb.  This is more useful
and arguably more intuitive too.

We need to remove test/performance/browsing_test.rb from the Rails app
being tested, otherwise "spring test" attempts to run this when it
iterates over test/**/*_test.rb, causing a failure due to ruby-prof
not being installed.

Signed-off-by: Adam Spiers <spring@adamspiers.org>
Fixed README for preload files configuration
Merge pull request #122 from miyagawa/fix-readme

Fixed README for preload files configuration
Merge pull request #102 from aspiers/test-multi-args

make "spring test" support any number of arguments
Added `Spring::Commands::Rake.environment_matchers`

This is for matching rake tasks to specific environments.

Closes #86.
Rails is not loaded for config/spring.rb
Copyediting the README

mostly the configuration section
Merge pull request #123 from turadg/patch-2

Copyediting the README
Print output when assertions fail

This should help with figuring out intermittent CI failures
Fix race condition

The pidfile could be deleted between when we check that it exists and
when we try to open it, causing an ENOENT error. So just try to open it
straight away, and rescue the ENOENT.