Add handler for values less than 1

[ryderstorm]

Before this change, the to_duration method returned an empty string ("") when it was called on a value less than 1. With this change, it will now return "Less than 1 second".

[coveralls]

Coverage remained the same at 100.0% when pulling 3626866 on ryderstorm:feature/add-handler-for-value-less-than-one into 581527f on digaev:master.

[digaev]

I'd rather use I18n instead of hard code this text.

[ryderstorm]

@digaev - thanks for guidance on this! It's my first code contribution to OSS.

And thanks for making the awesome gem, I use it all the time 🥇

[digaev]

@ryderstorm, thanks for contributing and pointing me to that issue, I wonder how could I miss it. Since you're using it so much notice that today I've released version 1.2, the main change is that an options (keys) of #to_duration method are now pluralized (year -> years etc.).