Only use files with the correct extension for migrations

lib/migrate.js

@@ -129,7 +129,10 @@ Migrate.prototype = {
         return Promise.promisify(fs.readdir, fs)(this.config.directory);
       })
       .then(function(migrations) {
-        return migrations.sort();
+        var ext = this.config.extension;
+        return _.filter(migrations, function (value) {
+          return value.indexOf(ext, value.length - ext.length) !== -1;
+        }).sort();
       });
   },
 

test/integration/migrate/test/random_bad_file.txt

@@ -0,0 +1 @@
+This is definitely not a migration.