doctor: warn about libcurl.framework

Addresses issue identified in https://github.com/Homebrew/homebrew/issues/32485#issuecomment-58614682.
All the spellings I see for libcurl.framework are lowercase. Do you mean to change the "libcurl.framework" string or to do something like:
```
found = Dir["/Library/Frameworks/*"].map { |framework| Pathname.new(framework).basename.to_s.downcase }
problematic_frameworks.select { |fmwk| found.include? fmwk.downcase }.map do |fmwk| <<-EOS.undent
  complaint here
  EOS
end.join
```
?