Remove unnecessary #encoding comments

These are no longer necessary as utf-8 is the default encoding in Ruby
2.
Remove .gitkeep files

Rails now uses .keep files instead of .gitkeep. Running the Rails 4
setup over the top of this existing repo means that I've now got the
.keep files.

I renamed a couple of the '.gitkeep' files to '.keep', where the
equivalent .keep file didn't exist.

I found the '.gitkeep' files with: `find . -name ".gitkeep" | grep -v
".bundle"`.
Remove vendor/plugins

This is no longer supported by Rails so I'm removing it.
Add rubocop to the development group

I've used this in the past to check my code against the Ruby Style
Guide[1]. I figure that it won't do any harm to keep it in the
development group of the repo.

[1] https://github.com/bbatsov/ruby-style-guide
Add newlines to the end of each file

As is good practice generally, and recommended in the Ruby Style Guide.