Fix deprecation

[Kolaru]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

This replaces the deprecated u_modified! with derivative_discontinuity!, as discussed #613

[Kolaru]

Rebased on master to fix conflicts.

[ChrisRackauckas]

Thanks!