Update 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.

2.6 and matplotlib seems to be not working with conda. :-(

Closes: yhat/ggplot#225

travis
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

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