Add $PREBUILD to cache signature (#405)

The buildpack should bust the cache whenever the PREBUILD boolean changes
to prevent the user from caching their local node_modules directory

Fixes #399

lib/cache.sh

 source $BP_DIR/lib/binaries.sh
 
 create_signature() {
-  echo "$(node --version); $(npm --version); $(yarn --version 2>/dev/null || true)"
+  echo "$(node --version); $(npm --version); $(yarn --version 2>/dev/null || true) $PREBUILD"
 }
 
 save_signature() {

test/fixtures/cache-prebuild/README.md

+A fake README, to keep npm from polluting stderr.
\ No newline at end of file

test/fixtures/cache-prebuild/package.json

+{
+  "name": "node-buildpack-test-app",
+  "version": "0.0.1",
+  "description": "node buildpack integration test app",
+  "repository" : {
+    "type" : "git",
+    "url" : "http://github.com/example/example.git"
+  },
+  "scripts": {
+    "start": "node server.js"
+  }
+}

test/run

@@ -68,6 +68,19 @@ testBuildWithCache() {
   assertCapturedSuccess
 }
 
+testCacheWithPrebuild() {
+  local cache=$(mktmpdir)
+  local env_dir=$(mktmpdir)
+  echo 'true' > "$env_dir"/PREBUILD
+
+  compile "cache-prebuild" $cache
+  assertCapturedSuccess
+
+  compile "cache-prebuild" $cache $env_dir
+  assertCaptured "Skipping cache restore (new-signature"
+  assertCapturedSuccess
+}
+
 testYarnSemver() {
   compile "yarn-semver"
   assertCaptured "Resolving yarn version ~0.17"