chore(package): update gulp-exec to version 3.0.1

Closes #47

Author: greenkeeper[bot]

package.json

@@ -37,7 +37,7 @@
     "gulp-changed": "^1.3.0",
     "gulp-changed-in-place": "^2.0.3",
     "gulp-cli": "^1.2.1",
-    "gulp-exec": "^2.1.2",
+    "gulp-exec": "^3.0.1",
     "gulp-print": "^2.0.1",
     "gulp-sourcemaps": "^1.6.0",
     "gulp-spawn-mocha": "^3.1.0",