Fix error: type `std::path::Display<'_,std::path::posix::Path>` does not implement any method in scope named `to_str`

Since `Path` implements `ToCStr`, it can be passed to Command::arg()
directly.
Fix error: type `Version` does not implement any method in scope named `to_str`
Fix error: use of moved value: `pcre_lib_path`
Fix error: type `pcre::CompilationError` does not implement any method in scope named `to_str`
Use the `rustup.sh` script with Travis

Described here: https://mail.mozilla.org/pipermail/rust-dev/2014-March/009223.html