Add AppSignal to ecosystem page

There's a new company offering monitoring services that are optimized for Rails in addition to New Relic and Scout: AppSignal.

I've added a link to AppSignal on the ecosystem page and I've replaced the image next to the monitoring tools text. The image still included Five Runs, they've discontinued their product a couple of years ago. The image now contains the logo of all three currently active companies.

While making this change I ran into a couple of minor snags which I've made changes for. The gem source has been converted to https and I've added a README file detailing how to start a server with this older version of Jekyll.