Remove hard dependency on sqlite3

Temping no longer relies on sqlite3 as a production dependency. By
moving it to a development dependency, Temping can be built on platforms
that don't build C extensions by default (I'm looking at you, jruby).

However, for build purposes, we should also do a check for jruby and add
`activerecord-jdbcsqlite3-adapter` as a development dependency instead
of `sqlite3`.

Signed-off-by: David Celis <me@davidcel.is>