Remove ignore for ie

IE stylesheets then render the following where %site-width-container is
used:

margin: 0 15px;
margin: 0 30px;
margin: 0 auto;

So the auto margin overrides the left and right margins which are
otherwise set.
Merge pull request #141 from gemmaleigh/remove-ignore-for-ie

Remove $ignore-for-ie: true to set auto margins