`hub` is crashing for me on a old CentOS setup with
"undefined method `shelljoin'".
On first use of the `git` function it is now checked if
`hub --version` returns successfully.
I've run into the issue where a stripped down container had no Ruby
installed, which caused `hub` to fail.
Therefore I've added an extra check to verify that `ruby` is in the
commands list before setting up this alias/function.
