Drop support for GHC 7 and ancient versions of `directory` and `time`

[andreasabel]

Towards

• 303

These commits have been individually verified by CI, so please preserve.

• Require GHC 8 and base >= 4.9 (released 2016-05-21)
• Require time >= 1.5 (released 2014-09-10)
• Require directory >= 1.2.2 (released 2015-03-18)

[andreasabel]

Well, it was basically grepping for #if and then going along. Bridging a couple of sleepless hours...