rbld.gemspec: Specify required cucumber version explicitely
rbld.gemspec: Specify required cucumber version explicitely

Otherwise it fetches the latest (3.0.x) via aruba dependencies and
test suite initialization fails due to broken backward compatibility.

Signed-off-by: Dmitry Fleytman <a8cf97adadec4e1b734a39bc5aea71b5741cfca1@daynix.com>

daynix/rebuild,daynix/rebuild,rbld/rebuild,daynix/rebuild,rbld/rebuild,rbld/rebuild