Don't include prettier in devDependencies

Fix node 6 & 8 builds

Author: nathan818fr

package.json

@@ -34,7 +34,6 @@
     "@types/jest": "^24.0.18",
     "@types/node": "^12.7.11",
     "jest": "^24.9.0",
-    "prettier": "^2.5.1",
     "rimraf": "^3.0.2",
     "ts-jest": "^24.3.0",
     "tslint": "^5.20.1",