Auto-fix XO version when still supporting Node.js 4

package.json

@@ -79,6 +79,6 @@
     "load-json-file": "^2.0.0",
     "nyc": "^8.1.0",
     "tempy": "^0.1.0",
-    "xo": "*"
+    "xo": "^0.20.3"
   }
 }

rules/xo.js

@@ -61,11 +61,14 @@ module.exports = ctx => {
 		installedVersion = installedVersion.replace(/^(~|\^)/, '');
 
 		const engine = pkg.engines && pkg.engines.node ? pkg.engines.node : undefined;
-		const supportsOlderVersions = engine && (semver.satisfies('0.10.0', engine) || semver.satisfies('0.12.0', engine));
+		const supportsNode0 = engine && (semver.satisfies('0.10.0', engine) || semver.satisfies('0.12.0', engine));
+		const supportsNode4 = engine && (semver.satisfies('4.0.0', engine));
 		const requiresUnicorn = requiredVersion === '*';
 
-		if (supportsOlderVersions && (!requiredVersion || requiredVersion === '*' || semver.gte(requiredVersion, '0.16.0'))) {
+		if (supportsNode0 && (!requiredVersion || requiredVersion === '*' || semver.gte(requiredVersion, '0.16.0'))) {
 			requiredVersion = '0.16.0';
+		} else if (supportsNode4 && (!requiredVersion || requiredVersion === '*' || semver.gte(requiredVersion, '0.20.3'))) {
+			requiredVersion = '0.20.3';
 		}
 
 		if (requiredVersion) {

test/fixtures/xo/support/node4/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "xo",
+  "engines": {
+    "node": ">=4"
+  },
+  "scripts": {
+    "test": "xo"
+  },
+  "devDependencies": {
+    "xo": "*"
+  }
+}

test/xo.js

@@ -233,6 +233,33 @@ test('support for older Node.js with unicorn version', async t => {
 	);
 });
 
+test('support for Node.js 4 with unicorn version', async t => {
+	await ruleTester(t, 'support/node4',
+		[
+			{
+				ruleId: 'xo',
+				severity: 'error',
+				message: 'Expected version \'0.20.3\' but found \'*\'.',
+				file: path.resolve(opts.cwd, 'support/node4/package.json')
+			}
+		],
+		[
+			{
+				name: 'xo',
+				engines: {
+					node: '>=4'
+				},
+				scripts: {
+					test: 'xo'
+				},
+				devDependencies: {
+					xo: '^0.20.3'
+				}
+			}
+		]
+	);
+});
+
 test('support for older Node.js with required unicorn version', async t => {
 	const ruleTester = clintonRuleTester(Object.assign({}, opts, {rules: {xo: ['error', '*']}}));