fix worker
simplify vnc server configuration

An array of arguments can be provided in addition to providing true/false and Class to the :vnc option.

eg.

JCukeForker::Runner.run Dir['features/**/*.feature'],
                       :max        => 4                                         # number of workers
                       :out        => "/path/to/reports",                       # output path
                       :format     => :html                                     # passed to `cucumber --format`,
                       :extra_args => %w[--extra arguments]                     # passed to cucumber,
                       :vnc        => %w[-geometry 1024x768 -depth 24 -dpi 96]  # passed to vncserver