Use atom_to_list in epcap:progname/0 to make Dialyzer in R15 happy

filename:join/1 accepts atoms in its argument, but because of an overly
restrictive type specification, Dialyzer gives a warning for this in
releases earlier than R16B.  As the R15 releases are still widely used,
it's worth putting in a call to atom_to_list to avoid false positives in
Dialyzer output.
Merge pull request #4 from legoscia/dialyzer-fix

Use atom_to_list in epcap:progname/0 to make Dialyzer in R15 happy