A couple of fixes in the test suite (tested with Ruby 2.1.0 and Rubinius 2.2.4)

* Test classes now inherits from MiniTest::Test instead of MiniTest::Unit::TestCase

* Require Celluloid autostart to make sure that the "Thread pool is
  not running error

 * Change assert false into assert true in the last test of TestDendriteSegment
   so that rake test stop failing