fix failure https://travis-ci.org/lino-framework/book/jobs/504732640
fix failure https://travis-ci.org/lino-framework/book/jobs/504732640

lino-framework/book,lsaffre/lino_book,lsaffre/lino_book,lino-framework/book,lino-framework/book,lsaffre/lino_book,lino-framework/book