Previous version of Respond was known to cause problems in IE8.
It was not necessary for developers who chose to write their CSS
in a 'mobile first' manner. Furthermore, there are some concerns
about the performance penalty Respond imposes upon legacy versions
of IE. If a future version of Respond is shown to be performant
and more reliable, it may be reconsidered for inclusion.
Along with this change is a simplification of the placeholder
Media Queries. Now there is only 1 Media Query example and it is
more clearly labelled as an example. The use of `em` units in the
condition is to reinforce the idea that Media Query breakpoints
should not be device-specific.