Fix coveralls badge config

[owayss]

This PR:

• d2db80b replaces String::split_off(usize) with String::truncate(usize) as its returned value is unused so that the tests no_entrypoint_fail and multiple_bin_fail have the expected output.

• 6fed7ed fixes the coveralls badge config so it uses the configured branch in the branch attribute.

Thanks for this crate! 🥳