Don't load spring/command until it is used, closes rails#457

lib/spring/client/run.rb

@@ -1,4 +1,3 @@
-require "spring/commands"
 require "rbconfig"
 require "socket"
 require "bundler"
@@ -139,6 +138,7 @@ def run_command(client, application)
         if pid && !pid.empty?
           log "got pid: #{pid}"
 
+          require "spring/commands"
           puts "Running via Spring preloader in process #{pid}" unless Spring.quiet
 
           forward_signals(pid.to_i)

lib/spring/test/acceptance_test.rb

@@ -244,6 +244,13 @@ def exec_name
         end
       end
 
+      test "spring commands when spring gem is missing" do
+        File.write(app.gemfile, app.gemfile.read.gsub(/gem 'spring.*/, ""))
+        app.run "bundle install"
+        assert_success "spring -v", stdout: "Spring version"
+        assert_success "spring help", stdout: "Usage: spring COMMAND [ARGS]"
+      end
+
       test "binstub when spring binary is missing" do
         begin
           File.rename(app.path("bin/spring"), app.path("bin/spring.bak"))