You probably haven't lost it, just put it in the wrong place. There are more sophisticated way of doing things but you should be able to just trawl up the RTClib folder and put it with all the other libraries, in the folder named libraries.
FWIW, you can get by without a library anyway. I found this method useful.
If you intend to use the DS1307 in the field, you may find the accuracy leaves a bit to be desired. This because of the lack of temperature compensation. The DS3231 costs about $1 more, is far more accurate, and can use exactly the same code.