Fix Plugin Issues

Several things changed/fixed here:

• unzip -j will unzip everything into a flat structure, removing sub-folders. This will not work here because Grafana validates the integrity of the plugins on loading (and expects the folder structure to be exact).
• Removed the pie chart plugin - this recently got added as a default plugin in Grafana so we don't need to add it here
• Updated the download link for the redis plugin to match the others (Github source)

For #2 (closed)