Add checkstyle to Gradle.

build.gradle

@@ -1,35 +1,39 @@
 apply plugin: 'java-library'
 apply plugin: 'maven'
+apply plugin: 'checkstyle'
 
 group = 'com.github.cs125-illinois'
 
 javadoc {
-  source = sourceSets.main.allJava
-  destinationDir = file('docs')
+    source = sourceSets.main.allJava
+    destinationDir = file('docs')
 }
 task sourcesJar(type: Jar, dependsOn: classes) {
-  classifier = 'sources'
-  from sourceSets.main.allSource
+    from sourceSets.main.allSource
+    classifier = 'sources'
 }
 
 task javadocJar(type: Jar, dependsOn: javadoc) {
-  classifier = 'javadoc'
-  from javadoc.destinationDir
+    from javadoc.destinationDir
+    classifier = 'javadoc'
 }
 
 artifacts {
-  archives sourcesJar
-  archives javadocJar
+    archives sourcesJar
+    archives javadocJar
 }
 
 test {
-  useTestNG()
+    useTestNG()
 }
 repositories {
-  jcenter()
+    jcenter()
 }
 dependencies {
-  testCompile 'org.testng:testng:6.10'
+    testCompile 'org.testng:testng:6.10'
+}
+checkstyle {
+    toolVersion "8.1"
 }