Fix ESLint ecmaVersion setting

The ecmaVersion setting should be a number, rather than a string, and
ESLint 6 will be stricter about parsing it.
diff --git a/.eslintrc.js b/.eslintrc.js
index 7381f85..ab17f62 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -13,7 +13,7 @@
     ecmaFeatures: {
       impliedStrict: true,
     },
-    ecmaVersion: '2019',
+    ecmaVersion: 2019,
     sourceType: 'module',
   },
   plugins: ['import', 'prettier'],