chore(.travis.yml): specify node_js version
chore(.travis.yml): specify node_js version
leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace,leanprover/ace