Drop strict dependency on Boost 1.55

This revers the dependency to 1.54. This can be done since
boost::coroutines has been made optional.

configure.ac

@@ -202,7 +202,7 @@ echo "==========================="
 
 
 dnl boost
-AX_BOOST_BASE([[1.55.0]])
+AX_BOOST_BASE([[1.54.0]])
 AX_BOOST_PYTHON
 if test "$BOOST_PYTHON_LIB" = ""; then
    AC_MSG_ERROR([No usable boost::python found])