Add requires-b2 check to top-level build file.

build.jam

@@ -3,6 +3,8 @@
 # (See accompanying file LICENSE_1_0.txt or copy at
 # http://www.boost.org/LICENSE_1_0.txt)
 
+require-b2 5.1 ;
+
 require-b2 5 ;
 
 import project ;