Add property slf4j.version

demo/app/pom.xml

 
     <properties>
         <main.class>org.pf4j.demo.Boot</main.class>
-        <slf4j.version>1.7.5</slf4j.version>
     </properties>
 
     <build>
                 <version>2.3</version>
                 <configuration>
                     <descriptors>
-                        <descriptor>
-                            src/main/assembly/assembly.xml
-                        </descriptor>
+                        <descriptor>src/main/assembly/assembly.xml</descriptor>
                     </descriptors>
                     <appendAssemblyId>false</appendAssemblyId>
                 </configuration>

pf4j/pom.xml

         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-api</artifactId>
-            <version>1.7.25</version>
+            <version>${slf4j.version}</version>
         </dependency>
         <dependency>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-log4j12</artifactId>
-            <version>1.7.25</version>
+            <version>${slf4j.version}</version>
             <scope>test</scope>
         </dependency>
         <dependency>

pom.xml

     <properties>
         <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
         <java.version>1.7</java.version>
+        <slf4j.version>1.7.25</slf4j.version>
 
         <junit.version>4.12</junit.version>
         <mockito.version>2.0.28-beta</mockito.version>