Run tests in Travis of each open source server

[michielbdejong]

webid-provider-tests

The following three open source Solid server implementations currently implement the webid-provider role:

• NSS
• PSS
• Nextcloud

Solid CRUD

The following four open source Solid server implementations currently implement the storage role with CRUD:

• NSS
• PSS
• Nextcloud
• CSS

WAC

The following three open source Solid server implementations currently implement the storage role with WAC:

• NSS
• PSS
• Nextcloud
• CSS