Specify node dependency using caret notation

This fixes an issue when deploying to CloudFoundry instances (such as GOV.UK PaaS) where the buildpack fails on the semver range:

```
   -----> Nodejs Buildpack version 1.6.28
   -----> Installing binaries
          engines.node (package.json): >=8.9.1 <9.0
          engines.npm (package.json): unspecified (use default)
          **WARNING** Dangerous semver range (>) in engines.node. See: http://docs.cloudfoundry.org/buildpacks/node/node-tips.html
          **ERROR** Unable to install node: improper constraint: >=8.9.1 <9.0
   Failed to compile droplet: Failed to run all supply scripts: exit status 14
```

Using ^8.9.1 instead which is semantically equivalent (any version greater than 8.9.1 but less than 9.0) allows the app to deploy successfully:

```
   -----> Nodejs Buildpack version 1.6.28
   -----> Installing binaries
          engines.node (package.json): ^8.9.1
          engines.npm (package.json): unspecified (use default)
   -----> Installing node 8.11.3
          Download [https://buildpacks.cloudfoundry.org/dependencies/node/node-8.11.3-linux-x64-34b80d71.tgz]
```

package.json

@@ -4,7 +4,7 @@
   "version": "7.0.0-beta.10",
   "private": true,
   "engines": {
-    "node": ">=8.9.1 <9.0"
+    "node": "^8.9.1"
   },
   "scripts": {
     "start": "node start.js",