now prefixing with '1' if string starts with recognized word but no number, ie. 'day'

This is a minor improvement that makes times better readable in a DSL I'm working on (using rufus scheduler and chronic duration). This will allow all of the below

every '2 hours 30min' 
every 'day' 
every 'minute'

etc in my DSL. 
no need to require the (novice) user to specify '1 minute' instead of just 'minute'. 
Clever gem btw, I like the implementation. I added a test as well...