Use jsDelivr instead of raw.githubusercontent.com

Try to fix download error on Solaris CI

https://rubyci.org/logs/rubyci.s3.amazonaws.com/unstable11x/ruby-master/log/20191121T162422Z.fail.html.gz
```
tool/downloader.rb:243:in `rescue in download': failed to download config.guess (RuntimeError)
Net::HTTPFatalError: 503 "Service Unavailable": https://raw.githubusercontent.com/gcc-mirror/gcc/master/config.guess
```

tool/downloader.rb

@@ -51,7 +51,7 @@ class Downloader
   class GNU < self
     def self.download(name, *rest)
       if https?
-        super("https://raw.githubusercontent.com/gcc-mirror/gcc/master/#{name}", name, *rest)
+        super("https://cdn.jsdelivr.net/gh/gcc-mirror/gcc@master/#{name}", name, *rest)
       else
         super("https://repo.or.cz/official-gcc.git/blob_plain/HEAD:/#{name}", name, *rest)
       end