Upgrade to libvips v8.9.0-rc4, drop support for Node.js 8

package.json

@@ -111,7 +111,7 @@
     "nan": "^2.14.0",
     "npmlog": "^4.1.2",
     "prebuild-install": "^5.3.3",
-    "semver": "^6.3.0",
+    "semver": "^7.1.1",
     "simple-get": "^3.1.0",
     "tar": "^5.0.5",
     "tunnel-agent": "^0.6.0"
@@ -126,7 +126,7 @@
     "license-checker": "^25.0.1",
     "mocha": "^6.2.2",
     "mock-fs": "^4.10.4",
-    "nyc": "^14.1.1",
+    "nyc": "^15.0.0",
     "prebuild": "^9.1.1",
     "prebuild-ci": "^3.1.0",
     "rimraf": "^3.0.0",
@@ -134,10 +134,10 @@
   },
   "license": "Apache-2.0",
   "config": {
-    "libvips": "8.8.1"
+    "libvips": "8.9.0-rc4"
   },
   "engines": {
-    "node": ">=8.5.0"
+    "node": ">=10.13.0"
   },
   "funding": {
     "url": "https://opencollective.com/libvips"