[Issue #47] CLI naming clash when run under Bundler

Fixes a load_path naming clash when `bin/approvals` is run under
Bundler.

Looks like it's picking up a different `CLI` lib from Thor instead of
the `Approvals::CLI`.

Verified by renaming `cli.rb` to something else & updating the `require`
statement to avoid naming clash.
Merge pull request #48 from zph/issue_47/fix_cli_naming_clash_under_bundler

[Issue #47] CLI naming clash when run under Bundler