This document will tell you how to upgrade from one version to one other.
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You need to install a HTTP client that provides the virtual package php-http/client-implementation.
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(Optinal) Add the client's service name to
cspoo_swiftmailer_mailgun.http_client. (Preferably with help from HttplugBundle) -
You might have to remove your bootstrap.php.cache, clear your apcu cache and then re-run composer update.
Read more about how to use HTTPlug here: http://php-http.readthedocs.io/en/latest/httplug/users.html