Upgrade to Gradle 2.13

This improves our documentation for the gradle protobuf plugin, as its
version is dependent on the gradle version.

Gradle now has the --tests flag, performance improvements, and support
for OpenPGP subkeys.
diff --git a/gradlew.bat b/gradlew.bat
index 72d362d..f6d5974 100644
--- a/gradlew.bat
+++ b/gradlew.bat
@@ -8,14 +8,14 @@
 @rem Set local scope for the variables with windows NT shell

 if "%OS%"=="Windows_NT" setlocal

 

-@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.

-set DEFAULT_JVM_OPTS=

-

 set DIRNAME=%~dp0

 if "%DIRNAME%" == "" set DIRNAME=.

 set APP_BASE_NAME=%~n0

 set APP_HOME=%DIRNAME%

 

+@rem Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.

+set DEFAULT_JVM_OPTS=

+

 @rem Find java.exe

 if defined JAVA_HOME goto findJavaFromJavaHome