Upgrade to Flyway 3.1

The 3.1 release includes an important fix to restore JDK 6
compatibility with the Gradle plugin [1].

Closes spring-projectsgh-2260

[1] flyway/flyway#779

Author: drumonii

spring-boot-dependencies/pom.xml

@@ -61,7 +61,7 @@
 		<commons-pool2.version>2.2</commons-pool2.version>
 		<crashub.version>1.3.0</crashub.version>
 		<dropwizard-metrics.version>3.1.0</dropwizard-metrics.version>
-		<flyway.version>3.0</flyway.version>
+		<flyway.version>3.1</flyway.version>
 		<freemarker.version>2.3.21</freemarker.version>
 		<gemfire.version>7.0.2</gemfire.version>
 		<glassfish-el.version>3.0.0</glassfish-el.version>