Upgrade extra-enforcer-rules 1.0-beta-7 -> 1.0-beta-9 (#31)

See:
- https://github.com/mojohaus/extra-enforcer-rules/releases/tag/extra-enforcer-rules-1.0-beta-8
- https://github.com/mojohaus/extra-enforcer-rules/releases/tag/extra-enforcer-rules-1.0-beta-9
- mojohaus/extra-enforcer-rules@extra-enforcer-rules-1.0-beta-7...extra-enforcer-rules-1.0-beta-9

pom.xml

@@ -75,7 +75,6 @@
     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <version.error-prone>2.3.1</version.error-prone>
-        <version.extra-enforcer-rules>1.0-beta-7</version.extra-enforcer-rules>
         <version.guava-beta-checker>1.0</version.guava-beta-checker>
         <version.javadoc>2.10.4</version.javadoc>
         <version.jdk>1.8</version.jdk>
@@ -158,11 +157,6 @@
                 <artifactId>javax.servlet-api</artifactId>
                 <version>3.1.0</version>
             </dependency>
-            <dependency>
-                <groupId>org.codehaus.mojo</groupId>
-                <artifactId>extra-enforcer-rules</artifactId>
-                <version>${version.extra-enforcer-rules}</version>
-            </dependency>
             <dependency>
                 <groupId>org.hamcrest</groupId>
                 <artifactId>java-hamcrest</artifactId>
@@ -477,7 +471,7 @@
                         <dependency>
                             <groupId>org.codehaus.mojo</groupId>
                             <artifactId>extra-enforcer-rules</artifactId>
-                            <version>${version.extra-enforcer-rules}</version>
+                            <version>1.0-beta-9</version>
                         </dependency>
                     </dependencies>
                 </plugin>