Rails 4/Ruby 2  support

For us at least, this fixes issue #15 in regards to Rails 4 and Ruby 2.0 support for this gem. I honestly have no idea why moving those two lines above the ````super```` call works, but for some reason after ````super```` is called master_connection and read_connections both become nil. When this happens, the error in #15 occurs. Moving the assignment above the ````super```` call somehow fixes it.

I'm not sure if this is caused by a change in Rails 4 or in Ruby 2.0 though.