fix Travis failure(ruby1.8.7): use 'rake/rdoctask' if 'rdoc/task' is missing
fix Travis failure(ruby1.8.7): use 'rake/rdoctask' if 'rdoc/task' is missing

orangain/review,suzuki/review,krororo/review,murashitas/review,kmuto/review,krororo/review,murashitas/review,takahashim/review,orangain/review,krororo/review,kmuto/review,kuad9/review_study,kuad9/review_study,kmuto/review,takahashim/review,suzuki/review,masawada/review,masawada/review