"only for use with JRuby" warnings thrown from bundler

[aaronfeng]

These gems require themselves in their gemspec files to get the version information:

• jdbc-derby
• jdbc-h2
• jdbc-hsqldb
• jdbc-jtds
• jdbc-mysql
• jdbc-postgres
• jdbc-SQLite3

See example at https://github.com/nicksieger/activerecord-jdbc-adapter/blob/master/jdbc-derby/jdbc-derby.gemspec#L4

Since bundler seems to load the gemspec files regardless of platform, this leads to "only for use with JRuby" warnings when running a jruby/mri project on MRI.

We could probably move the version information to a separate file and avoid this warning.

[nicksieger]

Sounds good. Do you mind sending a pull request?