If a default version and a regular version of etc are present at the
same time, RubyGems will end up duplicating work and running pristine
twice.

The `etc` gem is special because it's loaded by RubyGems by default.
When doing this, RubyGems will activate the regularly installed version.
The when `gem pristine` runs, it will find to installed specifications
but materialize both to the already activated specification.

Before:

```
$ gem pristine etc --version 1.4.3
Restoring gems to pristine condition...
Building native extensions. This could take a while...
Restored etc-1.4.3
Building native extensions. This could take a while...
Restored etc-1.4.3
```

After:

```
$ gem pristine etc --version 1.4.3
Restoring gems to pristine condition...
Skipped etc-1.4.3, it is a default gem
Building native extensions. This could take a while...
Restored etc-1.4.3
```

https://github.com/rubygems/rubygems/commit/5c279ac56b