gemspec: bump SQLite dependency

Forwarded: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=949824

SQLite in Debian was updated which makes dependency resolution fail. I would like to propose to make the version check less strict.

Edit: Probably, the safer approach is just bumping the dependency, but keeping ~>, instead of changing this to >=. Changed the proposed code, and the title of this issue, accordingly.