compile demo plugins for Java 8 (#302)

demo/plugins/pom.xml

@@ -16,7 +16,6 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
-        <java.version>1.7</java.version>
 
         <!-- Override below properties in each plugin's pom.xml -->
         <plugin.id />
@@ -28,14 +27,6 @@
 
     <build>
         <plugins>
-            <plugin>
-                <groupId>org.apache.maven.plugins</groupId>
-                <artifactId>maven-compiler-plugin</artifactId>
-                <configuration>
-                    <source>${java.version}</source>
-                    <target>${java.version}</target>
-                </configuration>
-            </plugin>
             <plugin>
                 <groupId>org.apache.maven.plugins</groupId>
                 <artifactId>maven-assembly-plugin</artifactId>