Give grid whitespace negater a unit (for IE8)

As per issue #11, give the font-size: 0 whitespace remover a unit so
that IE8 ignores it and still shows the content.

README.MD

@@ -139,7 +139,7 @@ Avalanche includes a media query mixin that makes use of the breakpoint aliases
 
 ## Browser support
 
-All modern browsers are supported and Internet Explorer from IE9 upwards. If IE8 support is required then all that is needed is a pixel-value to be set for `font-size` on `.grid__cell`. This is due to IE8 not supporting rem units. Use of [autoprefixer](https://github.com/postcss/autoprefixer) is recommended which will handle this for you automatically.
+All modern browsers are supported and Internet Explorer from IE9 upwards. If IE8 support is required then all that is needed is a pixel-value to be set for `font-size` on `.grid__cell`. This is due to IE8 not supporting rem units. Use of a 'rem-to-px' check during build is recommended.
 
 ## Questions
 

_avalanche.scss

@@ -160,7 +160,7 @@ $av-enable-grid-rev:          false !default;
   padding: 0;
   margin: 0;
   margin-left: -($av-gutter);
-  font-size: 0;
+  font-size: 0rem;
 }
 
   .#{$av-namespace}__cell{