Bugfix: __lt__ operator should compare with right-hand-side argument, not itself

Additionally `to_base_units` should be used instead of non-existant
`convert_to_base` to convert to base SI units.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Fix Travis-CI dependency install

Signed-off-by: Giel van Schijndel <me@mortis.eu>