fixed navbar.less for latest lessc version

[hvwaldow]

If the current version of less.js (2.5.1) compiles navbar.less (called from bin/less), the following error occurs:

An error occurred running the less command:
Command failed: NameError: #grid > .core > .span is undefined in /home/vonwalha/ckan/lib/default/src/ckan/ckan/public/base/vendor/bootstrap/less/navbar.less on line 199, column 3:
198 .navbar-fixed-bottom .container {
199   #grid > .core > .span(@gridColumns);
200 }

This results in an empty main.css.

Apparently this is due to an incompatibility between less.js versions. The fix was taken from
http://stackoverflow.com/questions/26628309/less-v2-does-not-compile-twitters-bootstrap-2-x

[brew]

We looked at this previously, and decided not to change the bootstrap vendor code, but instead to pin the less version in the docs. See the previous pull request for more details: #2101

[hvwaldow]

Thanks brew. Sorry, should have searched better through previous pull-requests.