update gcfg dependency to v1.4.0

[hairyhenderson]

Updating to the latest gcfg tag (see
src-d/gcfg#1 (comment)) so that go test will pass:

Before:

$ env GO111MODULE=on go test gopkg.in/src-d/go-git.v4/config

----------------------------------------------------------------------
FAIL: config_test.go:12: ConfigSuite.TestUnmarshall

config_test.go:40:
    c.Assert(err, IsNil)
... value scanner.ErrorList = scanner.ErrorList{(*scanner.Error)(0xc0000ca980)} ("16:9: unquoted '\\' must be followed by new line")

OOPS: 30 passed, 1 FAILED
--- FAIL: Test (0.00s)
FAIL
FAIL	gopkg.in/src-d/go-git.v4/config	0.036s

After:

$ env GO111MODULE=on go test gopkg.in/src-d/go-git.v4/config
ok  	gopkg.in/src-d/go-git.v4/config	0.037s

Signed-off-by: Dave Henderson dhenderson@gmail.com