Update gradle-download-task from 2.0.0 to 3.1.2 to fix boost zip down…

Summary:
Small pull request

 **motivation**
The doc 'Building React Native from source' is not up to date now.

While i try to build from the source my self, the gradle-download-task(2.0.0) will always fail to download boost zip with a failure of 403.

After i upgrade the gradle-download-task to 3.1.2, the download works well.

**Test plan (required)**
I tested to build the source myself, it works.

This is non-code update, it is document update, hope to help other people who get the same download failure.

**Code formatting**
Non-code update PR.
Closes facebook/react-native#11298

Differential Revision: D4288387

fbshipit-source-id: 5e6feef1ea80f7e3feddcc7e4abf2b5cea82f08f

build.gradle

@@ -7,7 +7,7 @@ buildscript {
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:2.2.2'
-        classpath 'de.undercouch:gradle-download-task:2.0.0'
+        classpath 'de.undercouch:gradle-download-task:3.1.2'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files

docs/AndroidBuildingFromSource.md

@@ -76,7 +76,7 @@ Add `gradle-download-task` as dependency in `android/build.gradle`:
 ...
     dependencies {
         classpath 'com.android.tools.build:gradle:1.3.1'
-        classpath 'de.undercouch:gradle-download-task:2.0.0'
+        classpath 'de.undercouch:gradle-download-task:3.1.2'
 
         // NOTE: Do not place your application dependencies here; they belong
         // in the individual module build.gradle files