Depend on ast 2.1.0+.
Depend on ast 2.1.0+.

whitequark/parser 2.2.3.0 depends on ast 1.1+, but we need features from
ast 2+. Addresses #172. Related: #173

glebm/i18n-tasks,bartimaeus/i18n-tasks,2947721120/i18n-tasks