Fix jdbc-derby warning when gemspec read by non-jruby
Fix jdbc-h2 warning when gemspec read by non-jruby
Fix jdbc-hsqldb warning when gemspec read by non-jruby
Fix jdbc-jtds warning when gemspec read by non-jruby
Fix jdbc-postgres warning when gemspec read by non-jruby
Fix jdbc-sqlite3 warning when gemspec read by non-jruby