Add auto_load support for Ruby 2.0

Ruby 2.0 completely removed Gem.source_index which was used in auto_load
to automatically find imageruby-* gems and load them. This behavior can
now be made using Gem::Specification.each.

Prefer Gem::Specification.each if Gem::Specification is defined and has
an each method. (1.8.7 defined Gem::Specification but without the each
method)

Fall back to Gem.source_index where available.