Scope VERSION constant within SlopeOne namespace.

Currently, `VERSION` is defined in the global namespace. It can collide with another global `VERSION` constant causing a constant redefinition warning. This commit moves it within the `SlopeOne` namespace to avoid this problem.

The pattern here (`lib/scope/version.rb`) is a common one. It's used, e.g., when you use bundler's gem skeleton generator.

One of the benefits of having the version constant in a separate file, is that it is possible to source the version in the gemspec without a chicken-egg problem. This branch makes that change also.