Replace usages of result_of() with tr1_result_of()

This fixes a bug in which boost::result_of() would return the
wrong result type for a function due to the new implementation
using decltype instead of the result_of protocol on compilers
that sufficently support C++11 (such as clang >= 3.2).

Now, boost::tr1_result_of() is used to explicitly request that
the result_of protocol be used even when decltype is supported
by the compiler.