tl: add version checks

[philips]

tl is going to need to be upgraded regularly as development continues. Figure out a way to notify users.

[philips]

• try out equinoix.io?
• add a /version/tl endpoint to the transparency log API?

[philips]

Implemented tl update with equinox.io.

Now, tl needs to warn users (via stderr) when they are running an out of date client.