Bump to latest stable version to fix bugs

[forddavis]

There are a few bugs with this gem's version of dropzone that are fixed in 5.9.3

For instance when file upload times out it shows "timedout after 30000 seconds" which should be "timedout after 30 seconds"

Would be awesome to have a version of this gem with the latest stable version without trying to replace assets using a rake task if possible.

[ncuesta]

Hi @forddavis, thanks for submitting this! I'm not actively using this gem, so I may miss important updates to the JS library.

I will try to release an updated version as soon as possible, but it would help a lot if you could create a PR with the latest JS library.

[forddavis]

@ncuesta Sounds good, I might take a stab at it.