Change output to default file instead of git

Once web is moved to 'oxidized-web' package and sqlite+git dependencies
are removed, it's lot easier to install 'oxidized-script' + 'oxidized'
on other machines which actually don't intend to run oxidized to collect
config
Now as we depend on puma + sqlite we need to compile native extensions,
which may be problematic and certainly unnecesassary just to run 'oxs'
Remove dependency to sequel, sqlite3 and grit

Not every installation will use git + sql. Try to give helpful error
message on missing dependencies.