Fix some lintian-reported issues by running lintian-brush

Changes include:

• Update standards version to 4.6.2, no changes needed. (lintian: out-of-date-standards-version)
• Bump debhelper from old 10 to 13. (lintian: package-uses-old-debhelper-compat-version)
• Use secure URI in Homepage field. (lintian: homepage-field-uses-insecure-uri)
• Use secure URI in debian/watch.
• Trim trailing whitespace. (lintian: trailing-whitespace)