Observed Errors in Solutions in d06 & d07

[Maeve Stites]

While going through excercises d06 and d07 I had noticed a few issues in reference to the solutions:

1. d06 Data: Deriving Quantities - The answer showed up on my copy of the downloaded course files for q4 and q5.
2. d07 Vis: Histograms - q4 doesn't have a solution.

[Zachary del Rosario]

Thanks for the feedback! Addressed in commit 4c566da