Revert #include ordering in SmilesMolSupplier.cpp

Recent commit 3358ec592580bae8a22faee90336f396db69de01 swapped around `#include <string>` and `include <boost/python.hpp>` in SmilesMolSupplier.cpp.

This caused compilation to fail on Mac OS X with errors related to `toupper`, `tolower`, `isupper`, etc. macros being redefined in pyport.h within the Python installation. From a Google search, I believe it is related to this problem:  http://bugs.python.org/issue10910

This commit simply swaps back `#include <string>` to come after `include <boost/python.hpp>`, which appears to solve the problem.