Revert "Allow using quotes around include"

[wallyqs]

Reverts #891 since it would mean not being able to use "include": as part of map. Only way that another file can be included would be via this syntax:

include "path/to/file"

and all the following would result in unknown field "include" error:

include: "path/to/file"
"include": "path/to/file"
'include': "path/to/file"
"include" "path/to/file"
'include' "path/to/file"

/cc @nats-io/core

[kozlovic]

LGTM

[coveralls]

Coverage increased (+0.09%) to 90.745% when pulling ff357d8 on revert-891-include-parsing-fixes into 2d0abd6 on master.