Don't abort for help display

`abort` is a quick way to print-and-quit, but asking for help isn't the
same as the command failing.

lib/boxen/cli.rb

@@ -18,7 +18,11 @@ module Boxen
     end
 
     def run
-      abort "#{flags}\n" if flags.help?
+      if flags.help?
+        puts flags
+        exit
+      end
+
       runner.run
     end
 

test/boxen_cli_test.rb

@@ -27,6 +27,8 @@ class BoxenCLITest < Boxen::Test
   end
 
   def test_help
+    $stdout.stubs(:write)
+
     flags = Boxen::Flags.new('--help')
     cli   = Boxen::CLI.new(@config, flags)
     cli.runner.expects(:run).never