added caching param to list.json requests

[emrysr]

adding the cache: true param to the js requests for list.json sends the current time as an additional parameter called _. this should stop older browsers from caching responses from the api as there will be a different value sent with each request.

eg: ?apikey=[XXXXXXXXX]&_=1577707087415

I cannot test with older versions of IE on windows. Could anybody test this on IE 8+ and let me know if this stops the caching...?

[emrysr]

https://community.openenergymonitor.org/t/my-solar-list-json-caching-on-internet-explorer/2896/4

has been tested on a windows machine by @jpalo in the above community post

[emrysr]

fix #127