Added an except for StaleElementReferenceException to is_element_visible logic

This is to address the intermittent failure as seen at http://qa-selenium.mv.mozilla.com:8080/job/remo.dev/476/testReport/junit/tests.test_people_page/TestPeoplePage/test_filter_results_by_name/

I believe that the element is changing in the middle of the call, which is resulting in the `StaleElementReferenceException`. Adding this to the `except` will allow Selenium to attempt to locate the element again.  

We did something similar to this in gaia-ui-tests recently.