fix(@embark/templates): ensure boilerplate template comes with valid whisper port

As part of the refactor in e330b338ea we've introduced a
second geth client process to enable whisper functionalities in DApps.
This introduced also a new default port for whisper (e330b338ea (diff-a61fbc84e4172487789d676437f26b5fR14)).
This default port has not been introduced on our boilerplate template which is
used in `embark new` when developers scaffold new apps.

This resulted in runtime errors where the geth process for whisper wasn't
able to successfuly boot up as its configured port address is already in use:

```
geth exited with error code 1
geth exited with error code 1
Blockchain process ended before the end of this process. Try running blockchain in a separate process using `$ embark blockchain`. Code: null
```

This commit changes the default port for whisper in the boilerplate template
to ensure apps created using `embark new` don't run into this error anymore.

dapps/templates/boilerplate/config/communication.js

@@ -11,7 +11,7 @@ module.exports = {
   development: {
     connection: {
       host: "localhost", // Host of the blockchain node
-      port: 8546, // Port of the blockchain node
+      port: 8557, // Port of the blockchain node
       type: "ws" // Type of connection (ws or rpc)
     }
   },