Update travis config

* add a new 2.6 travis config
* change the old_mpl config to not require old pandas (not sure
  why that doesn't work anaymore with conda) and use a two step
  to install fist mpl and then the rest (again no idea wha this
  works with conda but not installing mpl=1.1.1 and the rest
  together.

Closes: yhat/ggplot#225
Don't register all color modes

The utils.color module was registering all defined color models, which
was not needed and fails with older matplotlib (1.1.1) versions due to
old style classes (in old mpl) and new style "super" calls.

Closes: yhat/ggplot#227