bundle exec should be optional, per configuration setting

[morhekil]

Hi,

I was trying to use minitest mode on a ruby project that doesn't use bundler. There is a configuration option to disable "bundle exec" prefix for the commands, but it looks to me like it is not actually used, and all command are hardcoded to use bundler's prefix.

This change is to make it actually work, removing "bundle exec" if the relevant config option is set to nil.

[arthurnn]

thanks