[BUG] Breaking test showing that ENV-specified values cannot be overridden

A loquacious value specified via an ENV variable retains that value permanently.  It is not possible to change the value after the fact programmatically with a `app.field = 'value'` assignment later on.

This took me by surprise, as programmatically changing Loquacious configuration during the life-cycle of an application works okay _unless_ you specify an initial value via an ENV variable.

### Observed behavior
* The attempt to assign does not raise an exception
* Reading the configuration value yields the value specified in ENV.

### Desired behavior?
Perhaps this is intentional behavior, in order to cache and reduce hits to \#method_missing?  We should discuss whether it's best to fix or document this limitation.

### Work arounds
For now, users should probably employ an approach similar to [Adhearsion's SIGALRM toggling](https://github.com/adhearsion/adhearsion/blob/v2.5.4/lib/adhearsion/logging.rb#L56-L64), which is to copy/proxy the Loquacious configuration value to another entity rather than to change it.