Fixes deprecated File.exists?

Related info:
http://www.ruby-doc.org/core-2.1.1/File.html#method-c-exist-3F (In
"click to toggle source")
Let Travis test rvm 2.1.1
Merge pull request #208 from futhr/feature/deprecation-fixup

Fixes deprecated File.exists?
Merge pull request #210 from futhr/feature/rvm-2.1.1

Let Travis test rvm 2.1.1
Merge remote-tracking branch 'upstream/master'

Conflicts:
	lib/mini_magick/image.rb
	spec/lib/mini_magick/image_spec.rb