Use ltsv log format instead of plain text

[sue445]

Before

2016/09/27 20:14:31 http://example.com/1 [status 404] <nil>
2016/09/27 20:14:31 http://example.com/2 [status 404] <nil>

After

time:2016-09-27 20:12:44.761421981 +0900 JST    check_url:http://example.com/1  status:404      error:<nil>
time:2016-09-27 20:12:44.99323277 +0900 JST     check_url:http://example.com/2  status:404      error:<nil>

[coveralls]

Coverage remained the same at 64.039% when pulling 80d42b0 on feature/ltsv into 285b933 on master.

[coveralls]

Coverage remained the same at 64.039% when pulling f5da908 on feature/ltsv into 285b933 on master.