phase 2: mattes/migrate -> migratex (#848)

* move mattes migrations to migratex

* changes format of migrations to migratex format
* updates test runner to use new interface (double checked this with printlns,
the tests go fully down and then up, and work on pg/mysql)

* remove mattes/migrate

* update tests from deps

* update readme

* fix other file extensions

vendor/github.com/go-openapi/swag/json.go

@@ -96,8 +96,7 @@ func ConcatJSON(blobs ...[]byte) []byte {
 
 	last := len(blobs) - 1
 	var opening, closing byte
-	a := 0
-	idx := 0
+	var idx, a int
 	buf := bytes.NewBuffer(nil)
 
 	for i, b := range blobs {