title: Why I need add use_legacy_build_api: true when use gym build project?
body: <p>Why I need add use_legacy_build_api: true when use gym build project?</p>
  
  <p>I used Xcode 7.3 and gym 1.6.2,</p>
  
  <p>I build a new Projct(OC or swift is same),</p>
  
  <p>Below is error output:</p>
  
  <pre><code>2016-04-22 18:45:46.071 xcodebuild[135:10371572] [MT] PluginLoading: Required plug-in compatibility UUID F41BD31E-2683-44B8-AE7F-5F09E919790E for plug-in at path '~/Library/Application Support/Developer/Shared/Xcode/Plug-ins/BBUFullIssueNavigator.xcplugin' not present in DVTPlugInCompatibilityUUIDs
  2016-04-22 18:45:46.246 xcodebuild[135:10371572] ### Failed to load Addressbook class CNContactNameFormatter
  2016-04-22 18:45:46.300 xcodebuild[135:10371572] [MT] IDEDistribution: -[IDEDistributionLogging _createLoggingBundleAtPath:]: Created bundle at path '/var/folders/4w/fpkx9n7s3gnbcxfym8mqr18m0000gn/T/GymDemo_2016-04-22_18-45-46.299.xcdistributionlogs'.
  2016-04-22 18:45:46.585 xcodebuild[135:10371572] [MT] IDEDistribution: Step failed: &lt;IDEDistributionSigningAssetsStep: 0x7ff847a3e450&gt;: Error Domain=IDEDistributionErrorDomain Code=1 "(null)"
  error: exportArchive: The operation couldn’t be completed. (IDEDistributionErrorDomain error 1.)
  
  Error Domain=IDEDistributionErrorDomain Code=1 "(null)"
  
  ** EXPORT FAILED **
  [18:45:46]: Exit status: 70
  
  [!] Error packaging up the application
  </code></pre>