Dont replace npm because of lockfile if npm is already >= 5 (#499)

lib/binaries.sh

@@ -69,8 +69,11 @@ install_npm() {
   local version="$1"
   local dir="$2"
   local npm_lock="$3"
+  local npm_version="$(npm --version)"
 
-  if $npm_lock && [ "$version" == "" ]; then
+  # If the user has not specified a version of npm, but has an npm lockfile
+  # upgrade them to npm 5.x if a suitable version was not installed with Node
+  if $npm_lock && [ "$version" == "" ] && [ "${npm_version:0:1}" -lt "5" ]; then
     echo "Detected package-lock.json: defaulting npm to version 5.x.x"
     version="5.x.x"
   fi

test/fixtures/npm-lockfile-no-version/package.json

@@ -5,5 +5,8 @@
   "license": "MIT",
   "dependencies": {
     "lodash": "^4.16.4"
+  },
+  "engines": {
+    "node": "8.9.1"
   }
 }

test/fixtures/npm-lockfile-node-6-no-version/package-lock.json

+{
+  "name": "yarn",
+  "version": "1.0.0",
+  "lockfileVersion": 1,
+  "dependencies": {
+    "lodash": {
+      "version": "4.17.4",
+      "resolved": "https://registry.npmjs.org/lodash/-/lodash-4.17.4.tgz",
+      "integrity": "sha1-eCA6TRwyiuHYbcpkYONptX9AVa4="
+    }
+  }
+}

test/fixtures/npm-lockfile-node-6-no-version/package.json

+{
+  "name": "yarn",
+  "version": "1.0.0",
+  "main": "index.js",
+  "license": "MIT",
+  "dependencies": {
+    "lodash": "^4.16.4"
+  },
+  "engines": {
+    "node": "6.9.5"
+  }
+}

test/run

@@ -189,8 +189,16 @@ testYarnLockfileOutOfDate() {
 
 testDefaultToNpm5() {
   compile "npm-lockfile-no-version"
+  assertCaptured "Using default npm version"
+  assertCaptured "Installing node modules (package.json + package-lock)"
+  assertCapturedSuccess
+}
+
+testDefaultToNpm5WithNode6() {
+  compile "npm-lockfile-node-6-no-version"
   assertCaptured "Detected package-lock.json"
   assertCaptured "Bootstrapping npm 5"
+  assertCaptured "Installing node modules (package.json + package-lock)"
   assertCapturedSuccess
 }