Change plugin initialization to rails3 style

This addresses an issue where Paperclip can indefinitely block execution
of an application by telling ImageMagick to "identify -format %wx%h [0]"
which will then wait for input from STDIN forever.

Remove Rails 3.1 `read_inheritable_attribute` deprecation and add .rvmrc to gitignore.

Conflicts:

	lib/paperclip/storage/s3.rb
	test/storage_test.rb