Remove derivative dependency

[a1phyr]

Checklist

• n/a I've added tests for all code changes and additions (where applicable)
• n/a I've added a demonstration of the new feature to one or more examples
• n/a I've updated the book to reflect my changes
• n/a Usage of new public items is shown in the API docs

Derivative is a quite costly dependence in term of build time, so we can remove it at small cost

[a1phyr]

Do you need something else in order to merge this ?

[azriel91]

Depends -- I'm happy to merge incremental improvements like these (with one approval), but checking with the other members.

[azriel91]

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push