Only gem install sqlite3 on EL5, not EL6

For EL6, we want to continue using the ruby-sqlite3 package because it
works just fine.

Also, `rm ~/.gemrc` was missing -f, failing on machines that didn't have
a ~/.gemrc file.
Merge pull request #476 from nicklewis/storeconfigs-acceptance-fix

Only gem install sqlite3 on EL5, not EL6