Don't print traceback on EPIPE

Without this, you will get a traceback if you pipe to `head` for example:

```
$ ansible-lint ~/dev/surveymonkey/smstack/vagrant.yml | head
...
Traceback (most recent call last):
  File "/Users/marca/dev/git-repos/ansible-lint/py27.venv/bin/ansible-lint", line 10, in <module>
    execfile(__file__)
  File "/Users/marca/dev/git-repos/ansible-lint/bin/ansible-lint", line 78, in <module>
    sys.exit(main(sys.argv[1:]))
  File "/Users/marca/dev/git-repos/ansible-lint/bin/ansible-lint", line 73, in main
    print formatter.format(match)
IOError: [Errno 32] Broken pipe
```