Removing Throwable on ContainerExceptionInterface

[moufmouf]

We added Throwable on ContainerExceptionInterface because we thought this would not introduce any breaking changes.
Alas, some classes implementing the ContainerExceptionInterface are already adding the Throwable interface,
and PHP won't let us add the interface twice.

This problem occurs with PHP 7.2 and PHP 7.3 only.

See #30

[moufmouf]

Note: DO NOT MERGE THIS IN MASTER! (master is the 2.0 branch)
We need to create another branch from the 1.1.0 tag and merge this https://github.com/moufmouf/container/tree/fix/remove_throwable into the new branch.
Then, tag from the new branch.

[moufmouf]

Ok, I created a 1.1 branch in the psr/container repository and the PR is now correctly targetting this new 1.1 branch.
We can create the tag from this branch.

I'm not sure if we need to delete the branch later or if we keep the branch open for a future 1.1.2 (see #30 )