Use a CDN to download Boost

Summary:
Downloading from the CDN is much faster than from SourceForge, both on my home WiFi and at the office.

I checked using the `diff` utility that both files are identical.

**Test Plan**

Circle CI build on this PR.
Closes #11087

Differential Revision: D4226538

fbshipit-source-id: a30ec1d94fe3228342c4a198bf65df7a95e0c005

ReactAndroid/build.gradle

@@ -28,7 +28,7 @@ task createNativeDepsDirectories {
 
 task downloadBoost(dependsOn: createNativeDepsDirectories, type: Download) {
     // Use ZIP version as it's faster this way to selectively extract some parts of the archive
-    src 'https://downloads.sourceforge.net/project/boost/boost/1.57.0/boost_1_57_0.zip'
+    src 'https://unpkg.com/boost-mirror@1.57.0/boost.zip'
     // alternative
     // src 'http://mirror.nienbo.com/boost/boost_1_57_0.zip'
     onlyIfNewer true