Ensure watcher tests don't block during shutdown

These test failures from #93 inspired this change:
https://travis-ci.org/bitly/google_auth_proxy/jobs/62474406
https://travis-ci.org/bitly/google_auth_proxy/jobs/62474407

Both tests exhibited this pattern:
2015/05/13 22:10:54 validating: is xyzzy@example.com valid? false
2015/05/13 22:10:54 watching interrupted on event: "/tmp/test_auth_emails_300880185": CHMOD
2015/05/13 22:10:54 watching resumed for /tmp/test_auth_emails_300880185
2015/05/13 22:10:54 reloading after event: "/tmp/test_auth_emails_300880185": CHMOD
panic: test timed out after 1m0s

[snip]

goroutine 175 [chan send]:
github.com/bitly/google_auth_proxy.(*ValidatorTest).TearDown(0xc2080bc330)
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_test.go:27 +0x43
github.com/bitly/google_auth_proxy.TestValidatorOverwriteEmailListViaRenameAndReplace(0xc2080f2480)
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_watcher_test.go:103 +0x3b9

[snip]

goroutine 177 [chan send]:
github.com/bitly/google_auth_proxy.func·017()
        /home/travis/gopath/src/github.com/bitly/google_auth_proxy/validator_test.go:34 +0x41

I realized that the spurious CHMOD events were causing calls to
`func() { updated <- true }` (from validator_test.go:34), which caused
the goroutine running the watcher to block. At the same time,
ValidatorTest.TearDown was blocked by trying to send into the `done` channel.
The solution was to create a flag that ensured only one value was ever sent
into the update channel.