Update for mozilla/rust@a243360

std::util::ignore() was moved to `std::prelude` and renamed to drop().
Remove unused import `std::util`
Use feature `link_args`

https://travis-ci.org/cadencemarseille/rust-pcre/builds/15087702
https://travis-ci.org/cadencemarseille/rust-pcre/builds/15088496
Remove unused import `std::util`
Trap and handle the io_error condition when executing `pcre-config`

closes #6
Change 'self to 'a

Update for mozilla/rust#10889
Fix dead code warnings

https://travis-ci.org/cadencemarseille/rust-pcre/builds/15396978
Updates for std::run API changes

See mozilla/rust#10936
Replace uses of std::vec::raw::to_ptr() and to_mut_ptr()

See:
* mozilla/rust#10984 - Some general clean-up of std::vec::raw
* mozilla/rust@164f7a2 - std::vec: convert to(_mut)_ptr to as_... methods on &[] and &mut [].