Remove unnecessary module inclusion.

`SharedExampleGroup` needs to be extended, but
does not need to be included as well.

The specs had to be refactored a bit because they
declared shared examples in-line w/in an example,
which isn't a normal, supported use case.

Fixes #1042.
Fix 1.8.7 build.
Merge pull request #1069 from rspec/issue-1042

Remove unnecessary module inclusion.