Fix #737, place responsive css before bootstrap

The default config for concatenating the vendor css files does not seem to specify an order. However, for the responsive features of bootstrap to take effect, the file `bootstrap-responsive.css` must follow the file `bootstrap.css` in the source. This commit makes the files and their order explicit.

The default vendor config can be seen here:
https://github.com/testdouble/lineman/blob/master/config/files.coffee

This change has major influence on the way the site look on non-standard screen sizes.