[ios] reduce xcode version requirement to 7.0

Build bots only have 7.0.
diff --git a/packages/flutter_tools/lib/src/ios/mac.dart b/packages/flutter_tools/lib/src/ios/mac.dart
index 13c2258..0b83657 100644
--- a/packages/flutter_tools/lib/src/ios/mac.dart
+++ b/packages/flutter_tools/lib/src/ios/mac.dart
@@ -19,7 +19,7 @@
 String get homeDirectory => path.absolute(Platform.environment['HOME']);
 
 const int kXcodeRequiredVersionMajor = 7;
-const int kXcodeRequiredVersionMinor = 2;
+const int kXcodeRequiredVersionMinor = 0;
 
 class XCode {
   /// Returns [XCode] active in the current app context.