Add context classes to Composer autoload

In order to autoload the server context classes the "bootstrap"
directory was explicitly listed in Behat autoload configuration. This is
fine in the configuration of acceptance tests for the server, but it
would force the configuration of acceptance tests for the apps to
explicitly include the path for the server context classes to be able to
use them (for example, for the login step).

Besides with its own configuration Behat also supports autoloading
classes using Composer, so now context classes are autoloaded using
Composer instead; thanks to this the server context classes are
autoloaded also in the acceptance tests for apps without any explicit
configuration in them.

Signed-off-by: Daniel Calviño Sánchez <danxuliu@gmail.com>