README: Promote "Configuration" section to an <h2>
README: Promote "Configuration" section to an <h2>

This moves it out from under the "Installation" section.
* The "Installation" section is already pretty long
* The line under `<h2>`s in GitHub Markdown helps clarify that it's not a subsection of the "Bower" or "Node.js Mincer" sections
* Installation and configuration are arguably separate topics

thatsus/bs-sass,thatsus/bs-sass,bahrul221/bootstrap-sass,zBMNForks/bootstrap-sass,ruiii/bootstrap-sass,kyled09/splashPage,kyled09/splashPage,sonipalak/demo,twbs/bootstrap-sass,zBMNForks/bootstrap-sass,bahrul221/bootstrap-sass,thatsus/bs-sass,JasonRivers/bootstrap-sass,bahrul221/bootstrap-sass,CapeSepias/bootstrap-sass,smartexpose/twbs-bootstrap-sass,twbs/bootstrap-sass,smartexpose/twbs-bootstrap-sass,JasonRivers/bootstrap-sass,ruiii/bootstrap-sass,CapeSepias/bootstrap-sass,ruiii/bootstrap-sass,sonipalak/demo,JasonRivers/bootstrap-sass,CapeSepias/bootstrap-sass,JasonRivers/bootstrap-sass,sonipalak/demo,ruiii/bootstrap-sass,zBMNForks/bootstrap-sass,sonipalak/demo,twbs/bootstrap-sass,kyled09/splashPage,kyled09/splashPage,smartexpose/twbs-bootstrap-sass,zBMNForks/bootstrap-sass,thatsus/bs-sass,smartexpose/twbs-bootstrap-sass,twbs/bootstrap-sass,CapeSepias/bootstrap-sass,bahrul221/bootstrap-sass