De-emphasize Imagemagick in README

I noticed on Twitter that people were making the claim that Diffux uses
Imagemagick for diffing snapshots. That's not true, we use a combination
of `diff-lcs` and `chunky-png` to generate a our diff representations.
I've updated the README slightly to reflect this better.
De-emphasize PostgreSQL in README

PostgreSQL is not a hard requirement. SQLite is used by default and for
most people trying the application out, it is enough for the basic
use-case.