Improve check_is_writeable detection for group write permissions

[kontur]

FontsamplerHelpers->check_is_writeable returns true for this:

-rwxr-xr-x   1 johannes  _www  15517 Apr  3 14:47 fontsampler-css.css

Although for the _www group the file is, in fact, not writeable. This might be a marginal permissions setup, but is invalid nonetheless and results in a) the proper error warning about a failed less render write not being displayed and b) depending on log settings a php error warning is thrown.