Set version 3.2.0 (Forgotten after last release.)

Author: kazuki-ma

build.gradle

@@ -48,7 +48,7 @@ apply plugin: 'jacoco'
 apply plugin: 'com.github.ben-manes.versions'
 
 group = 'com.linecorp.bot'
-version = '3.1.0'
+version = '3.2.0'
 
 //set build variables based on build type (release, continuous integration, development)
 def isDevBuild