Allow building and publishing on different Java versions

Some users may need to consume the library on Java 8:
allow building and publishing with a different Java version.

Change-Id: I0c3001d286cdd22e02192f0c18906d99df5f5a1b
diff --git a/pom.xml b/pom.xml
index 1068902..a6d8f70 100644
--- a/pom.xml
+++ b/pom.xml
@@ -75,8 +75,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.6.1</version>
                 <configuration>
-                    <source>11</source>
-                    <target>11</target>
+                    <source>${java.specification.version}</source>
+                    <target>${java.specification.version}</target>
                 </configuration>
             </plugin>
             <plugin>