Remove unused gulp-exec dependency (#139)

Author: jonkoops

gulpfile.js

@@ -2,7 +2,6 @@
 
 var gulp = require('gulp');
 var jshint = require('gulp-jshint');
-var exec = require('gulp-exec');
 var stylish = require('jshint-stylish');
 var browserify = require('gulp-browserify');
 var uglify = require('gulp-uglify');

package-lock.json

@@ -31,7 +31,6 @@
     "gulp": "^4.0.0",
     "gulp-browserify": "^0.5.1",
     "gulp-coveralls": "^0.1.4",
-    "gulp-exec": "^3.0.1",
     "gulp-istanbul": "^1.1.3",
     "gulp-jshint": "^2.0.0",
     "gulp-mocha": "^5.0.0",