Revert changes for replica sets (tests were failing)

`ReplSetConnection` is unavailable which causes tests to break.

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Revert "Write a better test, making sure ReplSetConnection is passed options hash"
This reverts commit 27322365c01a72174e8e74637796772c31489cd7.

Revert "Added support for a Replica Set"
This reverts commit 8db1054a41c76a6fcd6cc3b382f408c2b4fc37b7.

Revert "Add test for replica set connections from config that specifies multiple hosts"
This reverts commit 7d724f3cf7544f31441c84cba880aeb13d8aab4c.
Update Gemfile to use https://rubygems.org
Make tests work on Ruby 1.9
Fix MongoMapper::Plugins.plugin for Ruby 1.9

The problem were `ClassMethods` defined on `Object`, which happens for example
when there `Delayed::Job` is available. The reason was the argument change for
`const_defined?` in Ruby 1.9, which is now used properly.