expose default in the description dsl. #156

Job	Run time
test (2.7, rails_edge)	41s
test (2.7, rack_edge)	15s
test (2.7, rack_3_0)	14s
test (3.0, rails_edge)	40s
test (3.0, rack_edge)	14s
test (3.0, rack_3_0)	10s
test (3.1, rails_edge)	48s
test (3.1, rack_edge)	14s
test (3.1, rack_3_0)	11s
test (3.2, rails_edge)	43s
test (3.2, rack_edge)	11s
test (3.2, rack_3_0)	10s
test (ruby-head, rails_edge)	32s
test (ruby-head, rack_edge)	16s
test (ruby-head, rack_3_0)	10s
test (truffleruby-head, rails_edge)	5m 5s
test (truffleruby-head, rack_edge)	38s
test (truffleruby-head, rack_3_0)	35s
test (jruby-head, rails_edge)	2m 36s
test (jruby-head, rack_edge)	52s
test (jruby-head, rack_3_0)	52s
finish	3s
	16m 10s

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