Fix prettier version

Author: cdimitroulas

package.json

@@ -29,7 +29,7 @@
     "cross-spawn": "^6.0.5",
     "eslint": "^6.2.2",
     "eslint-config-prettier": "^6.1.0",
-    "prettier": "^1.18.2",
+    "prettier": "1.18.2",
     "ts-node": "^8.3.0",
     "typescript": "^3.5.3",
     "yargs-parser": "^13.1.1"