Bugfix when try to define gruntfile property, and that has already been defined.

➜  projects  yo jhipster:entity teste                              

/home/eduardo/.nvm/v0.10.28/lib/node_modules/generator-jhipster/node_modules/yeoman-generator/lib/base.js:150
  Object.defineProperty(this, 'gruntfile', {
         ^
TypeError: Cannot redefine property: gruntfile
    at Function.defineProperty (native)
    at EntityGenerator.Base (/home/eduardo/.nvm/v0.10.28/lib/node_modules/generator-jhipster/node_modules/yeoman-generator/lib/base.js:150:10)
    at EntityGenerator.module.exports.Base.extend.constructor (/home/eduardo/.nvm/v0.10.28/lib/node_modules/generator-jhipster/node_modules/yeoman-generator/lib/named-base.js:19:10)
    at EntityGenerator.Generator (/home/eduardo/.nvm/v0.10.28/lib/node_modules/generator-jhipster/script-base.js:10:33)
    at new EntityGenerator (/home/eduardo/.nvm/v0.10.28/lib/node_modules/generator-jhipster/entity/index.js:11:14)
    at Environment.instantiate (/home/eduardo/.nvm/v0.10.28/lib/node_modules/yo/node_modules/yeoman-generator/lib/env/index.js:269:10)
    at Environment.create (/home/eduardo/.nvm/v0.10.28/lib/node_modules/yo/node_modules/yeoman-generator/lib/env/index.js:246:15)
    at Environment.run (/home/eduardo/.nvm/v0.10.28/lib/node_modules/yo/node_modules/yeoman-generator/lib/env/index.js:305:24)
    at init (/home/eduardo/.nvm/v0.10.28/lib/node_modules/yo/cli.js:113:7)
    at pre (/home/eduardo/.nvm/v0.10.28/lib/node_modules/yo/cli.js:131:3)