Upgrade to AssertJ-2.5.2 and Checkstyle-7.1

According changes in the `RequireThis`, the `this.` modifier is required now only in case of overlapping:
https://github.com/checkstyle/checkstyle/issues/3418

* Add `validateOnlyOverlapping = false` to reinstate the previous behavior, but we should think one more time if we really should be so strict with this rule

build.gradle

@@ -52,7 +52,7 @@ if (project.hasProperty('platformVersion')) {
 }
 
 ext {
-	assertjVersion = '3.4.1'
+	assertjVersion = '3.5.2'
 	servletApiVersion = '3.1.0'
 	slf4jVersion = '1.7.21'
 	springCloudAwsVersion = '1.1.1.RELEASE'
@@ -78,7 +78,7 @@ jacoco {
 
 checkstyle {
 	configFile = file('src/checkstyle/checkstyle.xml')
-	toolVersion = "6.16.1"
+	toolVersion = "7.1"
 }
 
 dependencies {

src/checkstyle/checkstyle.xml

@@ -68,6 +68,7 @@
 		<module name="MultipleVariableDeclarations" />
 		<module name="RequireThis">
 			<property name="checkMethods" value="false" />
+			<property name="validateOnlyOverlapping" value="false" />
 		</module>
 		<module name="OneStatementPerLine" />