Update easyrdf from 1.13.1

[joelit]

Description of the enhancement

Sweetrdf/easyrdf has a newer version that is not fully compatible with our code. There's a need for making the update, seeing what gets broken in phpunit, fixing those parts and being paranoid about the code that is not well covered by the tests.

Why is the enhancement important?

Fixing library dependencies early will be easier than fixing them late.