Pint: add some missing prefixes/units

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Merge branch 'pint/master'
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

 * Ensure proper shell commands
 * Disable testing NumPy 1.6 with Python 3.2/3.3
  - Apparently installation of NumPy consistently fails with this config

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