Fixed error in running shoulda context

Running shoulda context gives an error because method name [here](https://github.com/pgr0ss/vimux-ruby-test/blob/master/plugin/ruby.vim#L122) is enclosed in double quotes which is passed to send_to_vimux [here](https://github.com/pgr0ss/vimux-ruby-test/blob/master/plugin/ruby.vim#L149) which is also in double quotes.

This fix uses single quotes in method name for this vimux call to work:

```vim
Vim.command("call RunVimTmuxCommand(\"clear && #{test_command}\")")
```