Explicitly close HTTP server to avoid process.exit

Using `process.exit` directly can cause output to stderr/stdout to be
truncated, since it
[does not wait for output to streams to be sent](https://nodejs.org/api/process.html#process_process_exit_code).

This probably isn't an issue here for the PIP service, but it's worth
avoiding it since explicitly closing the HTTP server created by express
is easy, and will cause the process to quit "naturally", as nothing else
is running.

We did actually run into this issue in the [fuzzy-tester](pelias/fuzzy-tester#44)
and it's a pain to track down.

index.js

@@ -1,5 +1,6 @@
+const app = require('./app')(process.argv[2]);
+
 try {
-  const app = require('./app')(process.argv[2]);
   const port = ( parseInt(process.env.PORT) || 3102 );
 
   app.listen(port, () => {
@@ -8,6 +9,6 @@ try {
 
 } catch (err) {
   console.error(err);
-  process.exit(1);
-
+  process.exitCode = 1;
+  app.close();
 }