Limit the number of browser configurations tested by sauce by default.

We seem to be hitting job limits, probably because I've been
testing this hard all day.  So make passing the sauce tests
optional for now.

Add IE9 and iOS to the browser configuration set, since `promises-es6-tests`
version 0.5.0 fixes the strict mode issues.
See: https://github.com/promises-es6/promises-es6/pull/1