Skip non-footprint files in check_all script for travis ci

[ottojo]

This fixes KiCad/kicad-footprints#1545

[poeschlr]

Might it be better to explicitly check for the fp-lib-table such that we still get a warning if there is a file added that should not be there?

[ottojo]

That sounds useful indeed, i changed the script accordingly

[poeschlr]

thanks