Speed up finders/merge_requests_finder_spec
Speed up finders/merge_requests_finder_spec

Uses the :simple merge request factory trait introduced by d166e70; cuts
execution time of this spec in half.

8thcolor/eurucamp2014-htdsadr,8thcolor/rubyconfau2015-sadr,8thcolor/eurucamp2014-htdsadr,8thcolor/rubyconfau2015-sadr,8thcolor/eurucamp2014-htdsadr,8thcolor/rubyconfau2015-sadr