Fix Explore Map link text

The helper class 'visible-xs-inline' was added in Bootstrap 3.2
This backports that class temporarily, until we update Bootstrap

Fixes #1840
Well I've already got the PR to revert this up https://github.com/maurizi/OTM2/commit/775532132ecaa9a70c530c433ae534b06e0d4260, so this is probably a moot point