Remove unneeded require of `rspec/core/project_initializer`.

Each require takes time and it is a waste to require a file like this
that is so rarely used up front. Things that are rarely used should
be required at the point they are used rather than upfront.
Merge pull request #983 from rspec/remove_unneeded_require

Remove unneeded require of `rspec/core/project_initializer`.