GUACAMOLE-221: Update guacamole-common build to Java 1.8.

Without Java 1.8, we cannot add new functions to established interfaces
without breaking backward compatibility.

guacamole-common/pom.xml

@@ -57,14 +57,14 @@
     <build>
         <plugins>
 
-            <!-- Written for 1.6 -->
+            <!-- Written for 1.8 -->
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.3</version>
                 <configuration>
-                    <source>1.6</source>
-                    <target>1.6</target>
+                    <source>1.8</source>
+                    <target>1.8</target>
                     <compilerArgs>
                         <arg>-Xlint:all</arg>
                         <arg>-Werror</arg>