[skip ci] Fix minimal ruby version on README

According to rake.gemspec, rake required at least ruby 2.0.0

Author: esparta

README.rdoc

@@ -130,7 +130,7 @@ Rake is available under an MIT-style license.
 = Other stuff
 
 Author::   Jim Weirich <jim.weirich@gmail.com>
-Requires:: Ruby 1.9.3 or later
+Requires:: Ruby 2.0.0 or later
 License::  Copyright Jim Weirich.
            Released under an MIT-style license.  See the MIT-LICENSE
            file included in the distribution.