erlang r15x (and possibly earlier) don't put the correct path to

source files in the `compile` key of `module_info/1` so `rebar cover`
struggles locating source in some instances. in these cases a warning
is printed and crappy coverage info is written rather than none at all

src/rebar_prv_cover.erl

@@ -250,6 +250,9 @@ write_index_section(F, [{Section, DataFile, Mods}|Rest]) ->
     ok = file:write(F, "</table>\n"),
     write_index_section(F, Rest).
 
+%% fix for r15b which doesn't put the correct path in the `source` section
+%%  of `module_info(compile)`
+strip_coverdir([]) -> "";
 strip_coverdir(File) ->
     filename:join(lists:reverse(lists:sublist(lists:reverse(filename:split(File)),
                                               2))).