Use WAD for caching

This is the initial work on using [WAD](https://github.com/Fingertips/WAD) to speed up builds by caching downloaded modules to S3.

This uses a special version of WAD from [this pull request](https://github.com/Fingertips/WAD/pull/4).

Before this can be used, an S3 bucket and credentials must be created and encrypted with:

    travis encrypt S3_BUCKET_NAME=secretvalue S3_CREDENTIALS=accesskey:secretkey