fix flake8 version to 3.4.1

[seiro-ogasawara]

Fix flake8's version to 3.4.1.
flake8 3.5.0 makes failure to current datadogpy's code base.
(The failure of #234 is the same reason)

$ flake8 --max-line-length=100 datadog
datadog/dogshell/monitor.py:100:13: E722 do not use bare except'
datadog/dogshell/monitor.py:119:13: E722 do not use bare except'
datadog/dogshell/screenboard.py:177:9: E722 do not use bare except'
datadog/dogshell/screenboard.py:198:9: E722 do not use bare except'
datadog/dogshell/screenboard.py:274:9: E722 do not use bare except'
datadog/dogshell/timeboard.py:144:9: E722 do not use bare except'
datadog/dogshell/timeboard.py:235:9: E722 do not use bare except'
datadog/dogshell/timeboard.py:255:9: E722 do not use bare except'
datadog/threadstats/base.py:305:9: E722 do not use bare except'
datadog/threadstats/base.py:308:13: E722 do not use bare except'
datadog/threadstats/base.py:366:13: E722 do not use bare except'
datadog/threadstats/base.py:369:17: E722 do not use bare except'
datadog/threadstats/base.py:391:17: E722 do not use bare except'
datadog/threadstats/base.py:394:21: E722 do not use bare except'
datadog/util/hostname.py:126:5: E722 do not use bare except'

[yannmh]

🙇