changed the prepare command to npm install --force

Currently the prepare command runs the regular `npm install`. However this doesn't update already installed external dependencies. Therefore tests might succeed even though the latest version of a dependency has some changes which breaks the tests. This can result in exporting broken code. 

This change forces npm to check all dependencies for updates.
Merge pull request #2 from TiddoLangerak/patch-1

Changed the prepare command to npm install --force