[iOS] Fix ios build failed (#16)

Specific the vesion of libwebp to 1.0.2 to  fix Pod SDWebImage build failed due to libwebp 1.0.3
diff --git a/ios/Podfile b/ios/Podfile
index 46e047b..e9f2f0b 100644
--- a/ios/Podfile
+++ b/ios/Podfile
@@ -17,6 +17,7 @@
     pod 'SocketRocket', '0.4.2'
     pod 'ATSDK-Weex', '0.0.1',:configurations => ['Debug']
     pod 'BindingX', '1.0.3'
+    pod 'libwebp', '1.0.2'
     # WeexGcanvas is added by Weex Plugin, more info at https://market.dotwe.org/ext/list.htm
     #pod 'WeexGcanvas'
 end