Don't require linguist/repository

linguist/repository depends on the Rugged gem, which isn't included in the
github-linguist dependencies.

Applications and libraries using linguist/repository can still use it by
manually calling `require 'linguist/repository'`

This is option 1. Opening a parallel pull request to add a Rugged dependency.

This is my preferred approach since it doesn't add a new dependency, but
strictly speaking it also means that we need to bump the major version since
Linguist::Repository is no longer defined.

The reason for this change is that a 3rd-party with "require 'linguist'" gets a
load error for Rugged being missing since it is required in linguist/repository.