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
TST: update baseline image for new color schema

Closes: yhat/ggplot#210
convert dict comprehension to generator expression for 2.6

Closes: yhat/ggplot#100
Closes: yhat/ggplot#209
ENH: Only print warnings once

Closes: yhat/ggplot#121
REF: Remove included six and use the system one

Matplotlib requires six so we don't need to include our
own copy of six.

* Remove the included copy and all reference to it
* Make sure it is installed on travis
* Make sure it is installed when using setup.py

Closes: yhat/ggplot#226
Merge pull request #228 from JanSchulz/to-merge

Various fixes, especially getting the tests running again