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: ...
```