release: version 2.2.0

Author: Siroshun09

README.md

@@ -16,15 +16,15 @@ Event4J is the simple event library for Java.
     <dependency>
         <groupId>com.github.siroshun09.event4j</groupId>
         <artifactId>event4j</artifactId>
-        <version>2.2.0-SNAPSHOT</version>
+        <version>2.2.0</version>
     </dependency>
 ```
 
 #### For Gradle:
 
 ```
     dependencies {
-        compile 'com.github.siroshun09.event4j:event4j:2.2.0-SNAPSHOT'
+        compile 'com.github.siroshun09.event4j:event4j:2.2.0'
     }
 ```
 

pom.xml

@@ -30,7 +30,7 @@
 
     <groupId>com.github.siroshun09.event4j</groupId>
     <artifactId>event4j</artifactId>
-    <version>2.2.0-SNAPSHOT</version>
+    <version>2.2.0</version>
     <packaging>jar</packaging>
 
     <name>Event4J</name>