Upgrade cynic to v3

I released v3 of cynic a while back, figured I might as well contribute
an upgrade in triagebot.

The main changes that'll have an impact here are:

1. I've added the ability to pre parse schems in `build.rs`, which is
   now in use here.  This makes a big difference to the compile time
   of crates using large schemas (such as the GitHub schema triagebot
   uses).
2. The `schema_for_derives` attribute macro is no longer required so
   I've got rid of that.
3. Query variables can now be references so I've made that change in the
   places where it made sense.

Author: obmarg