Add only trusted projects' bin directory to $PATH

Assuming the binstubs for a project are in the local bin/ directory, you
can even go a step further to add the directory to shell $PATH so that
rspec can be invoked without the bin/ prefix:

    export PATH="./bin:$PATH"

However, doing so on a system that other people have write access to
(such as a shared host) is a security risk.

https://github.com/sstephenson/rbenv/issues/309