Added --force to rebuild native modules

[wyattjoh]

What does this PR do?

Adds a --force flag to the yarn install process so that when installing dependancies, we ensure that native modules are rebuilt.

How do I test this PR?

1. Build images locally: ./scripts/docker.sh
2. Create a new folder (elsewhere on your harddrive) and create a Dockerfile in it with the following content:

FROM coralproject/talk:latest-onbuild

3. Build the docker image: docker build -t coralproject/talk:pr-test .
4. Run the image docker run --rm -ti coralproject/talk:pr-test bash
5. Run the following node models/user.js

There should be an error (specifically about not connecting to the database), but you should no longer see an error related to bcrypt not being installed.

This issue is related to an upstream issue with yarn: yarnpkg/yarn#756

[jde]

Following the steps in the pr leads to a bcrypt error for me:

...
tmp :) ]$ docker run --rm -ti coralproject/talk:pr-test bash
root@b72f8b54aaff:/usr/src/app# node models/user.js
module.js:472
    throw err;
    ^

Error: Cannot find module '/usr/src/app/node_modules/bcrypt/lib/binding/bcrypt_lib.node'
    at Function.Module._resolveFilename (module.js:470:15)
    at Function.Module._load (module.js:418:25)
...