Hide spec var from gemspecs to silence warnings

This change prevents warnings to be emitted like:

```
bundler.gemspec:6: warning: shadowing outer local variable - spec
```

The gemspec's that emit the above warning is of the form:

```ruby
Gem::Specification.new do |spec| do
   # the actual spec
end
```

I think all of rubygems docs use examples where in the block var for a specification is `s`. This change will allow the use of the more natural `spec`.

Note that Bundler no longer uses `spec` as the block var to prevent that warning but IMHO gem authors should be able to use `spec` as the block var without getting a mysterious warning. :smile: