fix(travis): revert "run lint only on Node.js stable"
fix(travis): revert "run lint only on Node.js stable"

This reverts commit de1e16799709cba005432d6fa853b8e53b5ae064 because
textlint-rule-no-dead-link now supports Node.js v4 so that we do not
need (relatively) complex Travis CI setup.

textlint/textlint,textlint/textlint,textlint/textlint,textlint/textlint