### What
Unpin all non-stellar dependencies.

### Why
We have had reports that the libs are difficult to integrate into other
projects due to the rigid dependency requirements.

We ultimately need the dependencies to be pinned in stellar-core.

There's benefit to the tests in this repo running with the same
dependencies as what core pins to.

There's benefits to stellar-rpc also using the same dependencies so as
to decrease the chance that simulation runs with different behavior.
Also, the same applies to the SDK for test behavior.

Pinning the deps were an easy way to keep the dependencies consistent
everywhere, but it also makes it difficult for folks to use the env
libraries.

Close #1351

### Other things that need to happen

@graydon is making a tool to check that Cargo.lock files are using
consistent versions where possible across the env repo, core repo, and
we can also use that in the rpc repo. It won't be practical to expect
contract devs to use it on their contract projects, so contract devs
will be able to use whatever combination of dependencies and that's just
a limitation.

### Why not

We could say again that not doing this has greater benefits than
allowing folks to more easily use the libraries in their own projects.

(cherry picked from commit a54d997)