Remove --noheading flag from ag execution

The flag isn't needed, and seems to cause strange output.

Output with flag in vim:

app/models/group.rb|4| validates :yammer_group_id, :name, presence: true
||·
app/models/event.rb|24| validates :name, presence: ...

---

Output without:

app/models/event.rb|24| validates :name, presence: ...
app/models/event.rb|25| validates :name, length: ...
app/models/event.rb|26| validates :open, inclusion: ...