Moved endpoint version out of base_url.

Riot has different endpoint versions for different points.  For example,
champion is 1.1 but league is 2.1.  This allows each module to implement
their own endpoint version.

Unfortunately I should've implemented this before implementing the methods for `game` and `champions` as those will need to be modified slightly to add the version to their respective paths.

If you want to pull this in first, I'll fix my other two PRs to use the `endpoint_version`.  Then I'll write the methods for `league`.

Tests are green locally w/ this branch.