Revert Boost.Build change that breaks building for two Python versions.
    
    A change to tools/build/src/tools/python.jam means that the python=2.7
    argument to b2 is ignored and both libboost_python.so and
    libboost_python3.so are linked to libpython2.7.so. Reverting that change
    restores the previous behaviour that allowed building Boost.Python in
    two different ways.