Summary of changes:
7f6775f... Update VBox to 5.1.4 (*)
914b475... Force use VirtualBox-kmodsrc >= 5.1.4-3 (*)
c5463c8... Force use VirtualBox-kmodsrc >= 5.1.4-4, with fix for 4.8.0 (*)
0ba94aa... Update VBox to 5.1.6 (*)
(*) This commit already existed in another branch; no separate mail sent