Add !important keyword to float: none; rule of centered columns in grid system

Currently selectors:
[class*="column"] + [class*="column"]:last-child { float: $opposite-direction; }
[class*="column"] + [class*="column"].end { float: $default-float; }
are more specific (specifity= 3) than rules for centered columns (specifity=2), and if centered
column is the last one in a row it will use f.e. float: right; rule instead of
float: none; from centered classes

![issue_centered](https://cloud.githubusercontent.com/assets/1146430/2834376/14e5467e-cfdc-11e3-83f4-8bcceb452298.png)