Add faraday-multipart gem

We are getting the following notification when running tests:

```
To use multipart middleware with Faraday v2.0+, install `faraday-multipart` gem; note: this is used by the ManageGHES client for uploading licenses``
```

This commit adds the `faraday-multipart` gem (with `require: false`) to
clear the warning.

Gemfile

@@ -18,6 +18,7 @@ gem "csv"
 gem "delayed_job_active_record"
 gem "drb"
 gem "dynamic_form"
+gem "faraday-multipart", require: false
 gem "faraday-retry"
 gem "flutie"
 gem "font-awesome-rails"

Gemfile.lock

@@ -201,6 +201,8 @@ GEM
  railties (>= 5.0.0)
  faraday (2.9.2)
  faraday-net_http (>= 2.0, < 3.2)
+ faraday-multipart (1.0.4)
+ multipart-post (~> 2)
  faraday-net_http (3.1.0)
  net-http
  faraday-retry (2.2.1)
@@ -303,6 +305,7 @@ GEM
  msgpack (1.7.2)
  multi_json (1.15.0)
  multi_xml (0.6.0)
+ multipart-post (2.4.1)
  mustermann (3.0.0)
  ruby2_keywords (~> 0.0.1)
  mutex_m (0.2.0)
@@ -633,6 +636,7 @@ DEPENDENCIES
  dynamic_form
  email_spec
  factory_bot_rails
+ faraday-multipart
  faraday-retry
  flutie
  font-awesome-rails