pin go-swagger 0.7.4

This pins the version of go-swagger used, because
the results generated by different versions
can differ quite a bit (tested between 0.7.2 - 0.7.4),
and can cause CI / validation to fail.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Makefile

@@ -147,4 +147,4 @@ swagger-gen:
 		-w /go/src/github.com/docker/docker \
 		--entrypoint hack/generate-swagger-api.sh \
 		-e GOPATH=/go \
-		quay.io/goswagger/swagger
+		quay.io/goswagger/swagger:0.7.4