Upgrade to JavaMail 1.5.6

Closes gh-6929

Author: wilkinsona

spring-boot-dependencies/pom.xml

@@ -90,7 +90,7 @@
 		<janino.version>2.7.8</janino.version>
 		<javassist.version>3.18.1-GA</javassist.version> <!-- Same as Hibernate -->
 		<javax-cache.version>1.0.0</javax-cache.version>
-		<javax-mail.version>1.5.5</javax-mail.version>
+		<javax-mail.version>1.5.6</javax-mail.version>
 		<javax-transaction.version>1.2</javax-transaction.version>
 		<jaxen.version>1.1.6</jaxen.version>
 		<jaybird.version>2.2.10</jaybird.version>