fix following race error
fix following race error

==================
WARNING: DATA RACE
Write by goroutine 68:
  github.com/mackerelio/mackerel-agent/command.TestLoop.func1()
      /Users/Songmu/dev/src/github.com/mackerelio/mackerel-agent/command/command_test.go:242 +0x30
  github.com/mackerelio/mackerel-agent/command.TestLoop()
      /Users/Songmu/dev/src/github.com/mackerelio/mackerel-agent/command/command_test.go:345 +0x11c6
  testing.tRunner()
      /usr/local/Cellar/go/1.6.2/libexec/src/testing/testing.go:473 +0xdc

Previous read by goroutine 84:
  github.com/mackerelio/mackerel-agent/agent.(*Agent).Watch.func1()
      /Users/Songmu/dev/src/github.com/mackerelio/mackerel-agent/agent/agent.go:52 +0x17e

Goroutine 68 (running) created at:
  testing.RunTests()
      /usr/local/Cellar/go/1.6.2/libexec/src/testing/testing.go:582 +0xae2
  testing.(*M).Run()
      /usr/local/Cellar/go/1.6.2/libexec/src/testing/testing.go:515 +0x11d
  main.main()
      github.com/mackerelio/mackerel-agent/command/_test/_testmain.go:68 +0x210

Goroutine 84 (running) created at:
  github.com/mackerelio/mackerel-agent/agent.(*Agent).Watch()
      /Users/Songmu/dev/src/github.com/mackerelio/mackerel-agent/agent/agent.go:57 +0x87
  github.com/mackerelio/mackerel-agent/command.enqueueLoop()
      /Users/Songmu/dev/src/github.com/mackerelio/mackerel-agent/command/command.go:339 +0x72
==================

mackerelio/mackerel-agent,mackerelio/mackerel-agent,mackerelio/mackerel-agent