is_text_file(): bump allowable non-ASCII chars from 2% to 10%. Closes rhbz#922433

This change was tested to treat
os_release = "Fedora release 19 (Schrödinger's Cat)"
as text.
I feel this is a stop-gap measure, though...

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>