MODULES-1213 link pg_config binary into /usr/bin if not already in /usr/bin or /usr/local/bin

`pg_config` is required for many things that use the devel libraries, including most (all?) language bindings. If we're installing the -devel package and its bindir isn't `/usr/bin` or `/usr/local/bin`, symlink `pg_config` from the package's bindir into `/usr/bin`.

I also added vagrant-wrapper to the gemfile, as it seemed to be missing.