Trying to sort out the version numbering

Only tagged version on GitHub was 0.5.2, whereas there was a 0.5.3 on
the PyPi which was ahead of this tagged version, but not very close to
the current HEAD (25b282359409ef672ed9264c91d30ff969f05a96). So I'm
setting this as the 0.5.4 'release' to try and avoid confusion.