Fix chruby-exec sourcing chruby.sh

#### On commit

If I got it right, `chruby-exec` works as follows:
1. source `chruby.sh`
2. check arguments
3. execute `chruby {target_ruby} && $command` in a subshell

The problem is that sourced functions unlike environmental variables
are not 'inherited' by subshells.
(It is true at least in Ubuntu 12.04 LTS, where I tested this)

With "basic setup" you create `/etc/profile.d/chruby.sh` which sources `/usr/share/local/chruby/chruby.sh`
Then, when you invoke `chruby_exec` it works just fine, because
`/usr/share/local/chruby/chruby.sh` is sourced automaticaly by bash
through /etc/profile initialization script.

It would be great if you consider moving the process of sourcing
`/usr/share/local/chruby/chruby.sh` from the top of `chruby_exec` to a
command executed by a subshell. It would allow to use `chruby_exec`
without needing to source `/usr/local/chruby/chruby.sh` with shell init
scripts (`/etc/profile.d/chruby.sh`)

#### About sourced functions in subshells

You could check it with the following setup:

**set_func.sh**:

    function test_func()
    {
        0
    }

**get_func.sh**:

    source ./set_func.sh
    type test_func | head -1
    exec "$SHELL" -i -l -c "type test_func | head -1"

**and then**:

    $ ./get_func.sh
    test_func is a function
    bash: type: test_func: not found