dev-python/pymodbus: Update dependency from 'twisted' to 'twisted-core'

dev-python/pymodbus: Update dependency from 'twisted' to 'twisted-core'

The ebuild was renamed in the Portage tree on 09 Aug 2013:

  $ cat /usr/portage/dev-python/twisted-core/ChangeLog
  ...
  *twisted-core-13.0.0-r1 (09 Aug 2013)

    09 Aug 2013; Michał Górny <mgorny@gentoo.org> +twisted-core-13.0.0-r1.ebuild:
    Migrate to twisted-r1.

    06 Aug 2013; Michał Górny <mgorny@gentoo.org> twisted-core-12.2.0.ebuild,
    twisted-core-12.3.0.ebuild, twisted-core-13.0.0.ebuild:
    Fix patch paths. Bug #479942.

  *twisted-13.0.0 (08 Apr 2013)
  ...