Fix Faraday deprecation warning

This addresses the following deprecation warning:

    WARNING: `Faraday::Connection#authorization` is deprecated; it will be removed in version 2.0.
    While initializing your connection, use `#request(:authorization, ...)` instead.
    See https://lostisland.github.io/faraday/middleware/authentication for more usage info.

Faraday 2.x will support passing a username and password to the
middleware, and encoding it automatically, but versions prior to that do
not. Therefore, we encode it ourselves for backwards compatibility.

lib/bugsnag/api/client.rb

@@ -13,6 +13,8 @@
 require "bugsnag/api/client/stability"
 require "bugsnag/api/client/releases"
 
+require "base64"
+
 module Bugsnag
   module Api
 
@@ -169,9 +171,9 @@ def agent
           http.headers[:user_agent] = configuration.user_agent
 
           if basic_authenticated?
-            http.basic_auth configuration.email, configuration.password
+            http.request :authorization, "Basic", Base64.strict_encode64("#{configuration.email}:#{configuration.password}")
           elsif token_authenticated?
-            http.authorization "token", configuration.auth_token
+            http.request :authorization, "token", configuration.auth_token
           end
         end
       end