metadata-check: ignore GTK widgets if no display

[matthewrmshin]

Allow check to pass for GTK widgets if there is no display in the environment.

Problem reported by @scwhitehouse.

[matthewrmshin]

(Travis CI failure unrelated.)

[scwhitehouse]

I've checked this both in the easy test case, and in the real life use-case where I discovered this, and it does fix the issue, and to my knowledge the code change looks sensible too (though I'm happy to defer to an expert).

[sadielbartholomew]

The changes look suitable & valid so I am happy to approve given they resolve the issue as you have observed. Though perhaps we could trim the exception pair (220:227) in 'metadata_check.py' down to the following:

    except Exception as exc:
        # Ignore if there is no display for GTK widget
        if (isinstance(exc, RuntimeError) and not os.getenv('DISPLAY') and
                exc.args == ('could not open display',)):
            return
        return INVALID_IMPORT.format(widget_name, type(exc).__name__, exc)

[matthewrmshin]

Exception consolidated.

(I have also included the unrelated fix to Travis CI here.)