Use bundler for test-unit mode too.

So I see that in `rspec` mode, we already [assume bundler usage](https://github.com/r0man/ruby-test-mode/blob/master/ruby-test-mode.el#L281).

I did the same thing for test-unit mode too. While this makes it work, I have a couple of questions:

+ I see that we don't have tests for these command generating forms. I tried adding, but they use `file-name-directory` and `directory-file-name` - which check for the existence of these directories. So testing these commands is not easy - I tried this:

```el
(ert-deftest  ruby-test-test-command ()
  (should (equal nil (ruby-test-test-command "other/exists_test.rb"))))
```

Do you see any alternatives (newb here, new to elisp and ert. So guidance will be valuable).

+ It might be better for `ruby-test-test-command` and ` ruby-test-spec-command` to return the command without the `bundle exec` prefix. Then, `ruby-test-command` (which calls these two forms) can use a var, say `ruby-test-command-prefix`, as the prefix. By default, we can have this var set to `bundle exec ` and the user can turn it off, if they are not using bundler.

Do you think that's a valid use-case? If no one's going to use that, I'd rather keep it simple.