move a couple Swirl lessons to better match the EDA lectures #175

[I couldn't find in this repo where this would be specified, so perhaps it pertains more to do with setup in Coursera. Whatever the case, please pass these requests on to the applicable inbox.]

Course: 04_ExploratoryAnalysis

Request 1: Move the Swirl lesson "Working with Colors" from week 2 to week 3.
Request 2: Move the Swirl lesson "Clustering Example" from week 3 to week 4.

Thanks,
Steve