Upgrade immutables

This upgrades to the latest version of Immutables to work around the
issue of the javax.annotation package no longer being available on
new JDKs.

Author: io7m

net.adoptopenjdk.spi/src/main/java/net/adoptopenjdk/spi/ImmutableStyleType.java

@@ -35,6 +35,7 @@
   typeModifiable = "*Mutable",
   builder = "builder",
   build = "build",
+  allowedClasspathAnnotations = Override.class,
   visibility = Value.Style.ImplementationVisibility.PUBLIC,
   defaults = @Value.Immutable(copy = false))
 public @interface ImmutableStyleType

pom.xml

@@ -34,7 +34,7 @@
     <io7m.checkstyle.path>/net/adoptopenjdk/checkstyle/checkstyle.xml</io7m.checkstyle.path>
     <io7m.api.previousVersion>0.0.1</io7m.api.previousVersion>
 
-    <net.adoptopenjdk.org.immutables.version>2.6.0-alpha1</net.adoptopenjdk.org.immutables.version>
+    <net.adoptopenjdk.org.immutables.version>2.7.1</net.adoptopenjdk.org.immutables.version>
   </properties>
 
   <inceptionYear>2018</inceptionYear>