Updated Readme.md

Added manual installation instructions for `file.exe` on Windows
Add Rails 4.2 to Appraisals
v4.2.1
Update cocaine dependency

Cocaine changes how it returns the supplied path in 0.5.5. While this
has no effect on the workings of Paperclip in a normal capacity, it does
break the tests. This change fixes that and updates the cocaine
dependency to 0.5.5.