Bump B2 require to 5.2

Author: grafikrobot

build.jam

@@ -3,9 +3,7 @@
 # (See accompanying file LICENSE_1_0.txt or copy at
 # http://www.boost.org/LICENSE_1_0.txt)
 
-require-b2 5.1 ;
-
-import project ;
+require-b2 5.2 ;
 
 project /boost/lambda2
     : common-requirements