Bump `tslint-xo`

package.json

@@ -58,8 +58,8 @@
 		"license-webpack-plugin": "^1.1.1",
 		"lodash.isequal": "^4.5.0",
 		"nyc": "^11.2.1",
-		"tslint": "^5.8.0",
+		"tslint": "^5.9.1",
-		"tslint-xo": "^0.4.0",
+		"tslint-xo": "^0.5.0",
 		"typedoc": "^0.9.0",
 		"typescript": "^2.6.2",
 		"vali-date": "^1.0.0",

source/lib/predicates/predicate.ts

@@ -27,7 +27,7 @@ export const validatorSymbol = Symbol('validators');
 export class Predicate<T = any> {
 	constructor(
 		type: string,
-		private context: Context = {
+		private readonly context: Context = {
 			validators: []
 		}
 	) {