Remove default command entirely

Since this has been a source of confusion and many support tickets it
was decided to require `cmd:` to be explicitly set by the user, this is
the first step in that process.

Signed-off-by: Tyler Pickett <t.pickett66@gmail.com>
update Guardfile template to include cmd

Signed-off-by: Tyler Pickett <t.pickett66@gmail.com>
Refuse to run unless the cmd option is specified

After much discussion on the topic and many support questions about
bundler not being used it was decided that the default behavior should
be to not run any specs unless the user has specified exactly how they
should be run. In addition to warning the user of the change a sensible
default has been added to the Guardfile template.

Signed-off-by: Tyler Pickett <t.pickett66@gmail.com>
Update README to reflect changes in cmd option

Signed-off-by: Tyler Pickett <t.pickett66@gmail.com>
Merge pull request #270 from tpickett66/default-command

Made :cmd option required