make received_messages pretty_printable
delete unused Target class from Spy
Test with unstable ruby versions
latest ruby version is 2.1.3 in 2.1.x
make spies stoppable

Now spies (Spy and ClassSpy) can stop logging their recived messages.
This is useful when you avoid to log unexpectedly.
I need this feature because ClassSpy unexpectedly logs its receivers'
"==" method inspect method in its test code.
ignore rubinius's build result

it happenes syntax error due to a bug.
I have to report.