update gulp-git version

It uses an old version of require.dir, which is broken in node 8

Author: sokra

package.json

@@ -38,7 +38,7 @@
     "gulp-eslint": "^1.1.1",
     "gulp-espower": "^1.0.2",
     "gulp-filter": "^3.0.1",
-    "gulp-git": "^1.6.1",
+    "gulp-git": "^2.4.0",
     "gulp-mocha": "^2.2.0",
     "gulp-plumber": "^1.0.1",
     "gulp-sourcemaps": "^1.6.0",