Upgrade to Assertj 3.8.0

Closes gh-9308

spring-boot-dependencies/pom.xml

@@ -51,7 +51,7 @@
 		<appengine-sdk.version>1.9.53</appengine-sdk.version>
 		<artemis.version>1.5.5</artemis.version>
 		<aspectj.version>1.8.10</aspectj.version>
-		<assertj.version>3.7.0</assertj.version>
+		<assertj.version>3.8.0</assertj.version>
 		<atomikos.version>3.9.3</atomikos.version>
 		<bitronix.version>2.1.4</bitronix.version>
 		<byte-buddy.version>1.6.14</byte-buddy.version>