Bump version for next beta release

And update the CHANGELOG.
Deprecate Opal::Sprockets::Environment

It can easily be replaced by `Opal::Server` or by appending `Opal.paths` 
to a `Sprockets::Environment`:

    Sprockets::Environment.new.tap { |e| Opal.paths.each {|p| e.append_path(p)} }
Add a failing test case for subfolders sourcemaps

for #648
Use File.basename to get correct path