Use Cocoapods CDN for pod install

[chenxiao0228]

https://blog.cocoapods.org/CocoaPods-1.7.2/

It is supported since Cocoapods 1.7.2, and can drastically improve pod install speed. Let's also bump cocoapods to the latest.

diff --git a/Podfile b/Podfile
index fb20817..b14894e 100644
--- a/Podfile
+++ b/Podfile
@@ -1,4 +1,4 @@
-source 'https://github.com/CocoaPods/Specs.git'
+source 'https://cdn.cocoapods.org/'
 
 platform :ios, "10.0"

[chenxiao0228]

@bkoatz FYI