Gradle wrapper: upgrade to Gradle 5.2.1

We have configured our Gradle wrapper to use Gradle 4.8.1 as the version
used for executing the build.

Gradle 5.2.1 has been announced for a while and it gives better support
for JDK11[1], which we may wish to upgrade to in the future.

To keep us updated with Gradle's development, let's upgrade to Gradle
5.2.1.

The Gradle wrapper upgrade is performed by running the command:

    ./gradlew wrapper --gradle-version 5.2.1

Also, update any references to the Gradle version we are using in the code
base to 5.2.1.

[1] https://docs.gradle.org/5.2.1/release-notes.html

build.gradle

@@ -1,6 +1,6 @@
 // Gradle Configuration File
 // For more details take a look at the Java Quickstart chapter in the Gradle
-// user guide available at http://gradle.org/docs/4.8.1/userguide/tutorial_java_projects.html
+// user guide available at http://gradle.org/docs/5.2.1/userguide/tutorial_java_projects.html
 
 import org.gradle.api.tasks.testing.logging.TestLogEvent
 

gradle/wrapper/gradle-wrapper.properties

@@ -1,5 +1,5 @@
 distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-4.8.1-bin.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-5.2.1-bin.zip
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists

gradlew

@@ -28,7 +28,7 @@ APP_NAME="Gradle"
 APP_BASE_NAME=`basename "$0"`
 
 # Add default JVM options here. You can also use JAVA_OPTS and GRADLE_OPTS to pass JVM options to this script.
-DEFAULT_JVM_OPTS=""
+DEFAULT_JVM_OPTS='"-Xmx64m"'
 
 # Use the maximum available, or set MAX_FD != -1 to use that value.
 MAX_FD="maximum"

gradlew.bat

@@ -14,7 +14,7 @@ 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=
+set DEFAULT_JVM_OPTS="-Xmx64m"
 
 @rem Find java.exe
 if defined JAVA_HOME goto findJavaFromJavaHome