Scope VERSION constant within SlopeOne namespace.

Previously, VERSION was defined in the global namespace. It could collide with
another global VERSION constant causing a constant redefinition warning.

With the version constant in a separate file, it is also possible to source the
version in the gemspec without a chicken-egg problem.
Merge pull request #1 from rsutphin/scope_version_constant

Scope VERSION constant within SlopeOne namespace.