Update wikimedia/assert, minimum PHP version

Upgrade wikimedia/assert to 0.4.0 (no major changes but needed to
avoid incompatible dependencies in mediawiki/vendor). Upgrade PHP
requirement to 7.x or HHVM (needed by wikimedia/assert 0.4 and
the end-of-life for 5.6 was January 1, anyway).

Author: tgr

.travis.yml

@@ -3,7 +3,6 @@ language: php
 dist: trusty
 
 php:
-  - 5.6
   - 7.0
   - 7.1
   - 7.2

composer.json

@@ -23,9 +23,9 @@
 		"irc": "irc://irc.freenode.net/wikidata"
 	},
 	"require": {
-		"php": ">=5.6",
+		"php": ">=5.6.99",
 		"data-values/data-values": "~0.1|~1.0|~2.0",
-		"wikimedia/assert": "~0.2.2"
+		"wikimedia/assert": "~0.4.0"
 	},
 	"require-dev": {
 		"ockcyp/covers-validator": "~0.5.0",