Do not install lxml > 3.4.0

After running a ``pip install -U``, I discovered that lxml doesn't
compile with gcc 4.8.3.  The error is:

    x86_64-linux-gnu-gcc: error: unrecognized command line option ‘-fstack-protector-strong’

Peg the upper limit until this is solved upstream.
Use python-rightscale from pypi