Revert "Use Gemfury as the first port of call for bundler."

This reverts commit 1171cf29af3a1c5ea88e2584ad3c8b8530b7b1b8.

It appears that Bundler reversed the order of precendence on sources
some time between 1.3.5 and 1.5.1, so this order is incorrect for the
version used by the majority of GDS systems, 1.3.5.

This will need to be switched back as and when we update Bundler across
the board.