Do not load library code from gemspec to prevent warning in non-jruby

See https://github.com/bundler/bundler/issues/2482

Under mri 1.9.3, whenever we ran a bundle command or loaded rails, we'd get the warnings 

    Jdbc-Derby is only for use with JRuby
    Jdbc-H2 is only for use with JRuby
    Jdbc-HSQLDB is only for use with JRuby
    Jdbc-JTDS is only for use with JRuby
    Jdbc-MySQL is only for use with JRuby
    Jdbc-Postgres is only for use with JRuby
    Jdbc-SQLite3 is only for use with JRuby

This was because the driver gemspec would unnecessarily load the whole driver code e.g. https://github.com/jruby/activerecord-jdbc-adapter/blob/fdde15de6ee823db04f2d827b3a97d6b61c48afb/jdbc-mysql/jdbc-mysql.gemspec#L4 

This PR fixes that