Fix default exclusion of vendor directories

Found out in discussion around #900 that @fshowalter had to add his own configuration to exclude `vendor`.

Excluding directories from a `.rubocop.yml` file works, but before this fix, the setting of `AllCops`/`Excludes` in `default.yml` didn't have any effect. I've found that this bug has existed ever since `vendor/**` was added in `default.yml`.

The problem was that `Excludes` paths from `default.yml`, and other files not named `.rubocop.yml`, were not made absolute. They need to be since `AllCops`/`Excludes` has special handling and is always matched with an absolute path.