Histogram TypeError

- log an error to console if the wrong data type is passed to moz_chart for a histogram. Currently, a cryptic error from d3 pops up and the rest of the script fails to run.

- added http prefix to two source tags in index.html so I could test things on my machine. Not sure if there's a reason why they were missing the prefix.
Yes, let's explicitly add https to those two URLs then and change the d3.js url to https://cdnjs.cloudflare.com/ajax/libs/d3/3.4.11/d3.min.js