Add /bin to Exec paths

While trying to get an Ubuntu 12.04.2 LTS Virtualbox VM using puppet-rbenv up and running, I got this error, complaining that `Exec[rbenv::plugin::update <user> ruby-build]` couldn't find sed:

```
Notice: /Stage[main]//Rbenv::Compile[1.9.3-p392]/Rbenv::Plugin::Rubybuild[rbenv::rubybuild::larry]/Rbenv::Plugin[rbenv::plugin::rubybuild::larry]/Exec[rbenv::plugin::update larry ruby-build]/returns: /usr/lib/git-core/git-pull: 74: /usr/lib/git-core/git-sh-setup: sed: not found
```

It also complained about not being able to find uname (both sed and uname are called from `git-sh-setup`).

It looks like the problem is that there are a few Exec's that have a `path` that doesn't include `/bin`, which is where sed and uname live, at least for this version of Ubuntu. 

This pull request adds `/bin` to the path for the three Exec's that have `path` parameters which contain "the usual suspects" (e.g. `/usr/bin`, `/usr/sbin`, etc). That fixed it for me.