Fixed selenium failure.

This fixes (at least according to tests on the ci itself)
admin_widgets.tests.RelatedFieldWidgetSeleniumFirefoxTests.test_foreign_key_using_to_field

(http://ci.djangoproject.com/job/Django/database=mysql_gis,python=python2.7/3792/testReport/junit/admin_widgets.tests/RelatedFieldWidgetSeleniumFirefoxTests/test_foreign_key_using_to_field/)

The cause for this issue seems to be that wait_page_loaded was executed before click fired
and as such no profile got saved (again just an educated guess, but with this fix I can no
longer reproduce it -- fingers crossed).
Added build to flake8.exclude.