Follow rails pattern for optional dependency gems

[cjcolvar]

See discussion on #94

Try doing something more like rails / valkyrie to load gems for adapters and warn users when they aren't installed and attempting to use them.
https://github.com/samvera/valkyrie/blob/1ad73aed5b41aa1957912f198015629e05a2a2b1/lib/valkyrie/persistence/fedora.rb#L3-L9

[jrochkind]

Oh I see you already have a ticket on this!

For a concrete example of how it ends up working now, see documentation in #101

There are other possible alternatives:

• We could just list all these dependencies as actual dependencies? But I imagine the reason not to include them in the formal dependencies is to not force every app using active_encode to install them, even if they'll never use the one adapter that needs them.

• We could put each adapter (at least if it needs additional dependencies) in it's own gem, say active_encode-mediaconvert. But I suspect the added maintenance burden of multiple gems that all need to work with each other as they each progress -- is more than we can handle right now.

So something like the Rails/valkyrie approach is probably the least bad low hanging fruit improvement.

I think it is preferable to expand on what valkyrie does, and formally specify the version specification (1.x, 2.x etc) of the "optional dependency" that is required. I think Rails does do that, although its' error message has in the past confusingly not told you what version it needed! I wrote more about this here: https://bibwild.wordpress.com/2015/09/09/optional-gem-dependencies/