Make signature ASCII armored

 + It's better for portability to output ASCII armored signature. It's also
 advised by the Apache Release Signing Guide, availaible here.
 http://www.apache.org/dev/release-signing.html#signing-basics
 Furthermore, even if ASCII armored output is much bigger (more than twice), we
 speak of a few hundred Bytes.

sign-dist.sh

@@ -17,6 +17,6 @@ do
   sha256sum ${element} > ${element}.sha256
 
   echo "Signing" ${element}
-  gpg2 --detach-sign ${element}
+  gpg2 --armor --detach-sign ${element}
 done