Skip rdoc to speed up Ruby compilation

doc/install/installation.md

@@ -93,7 +93,7 @@ Download Ruby and compile it:
     mkdir /tmp/ruby && cd /tmp/ruby
     curl --progress ftp://ftp.ruby-lang.org/pub/ruby/2.0/ruby-2.0.0-p247.tar.gz | tar xz
     cd ruby-2.0.0-p247
-    ./configure
+    ./configure --disable-install-rdoc
     make
     sudo make install