Fix some issues reported by lintian

Fix some issues reported by lintian

• Use secure URI in Homepage field. (homepage-field-uses-insecure-uri)
• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

This merge proposal was created by the Janitor bot, and it will automatically rebase or close this proposal as appropriate when the target branch changes. Any comments you leave here will be read by the Janitor's maintainers.

Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/mininet/b1a4b237-339a-478b-b8bf-b951a110eea3.

Debdiff

These changes affect the binary packages:

File lists identical (after any substitutions)

Control files of package mininet: lines which differ (wdiff format)

• Homepage: [-http​://mininet.org/-] {+https​://mininet.org/+}

No differences were encountered between the control files of package **mininet-dbgsym**

You can also view the diffoscope diff (unfiltered).