Use kotlin 1.3.20 intead of 1.3.11 (#322)
Use kotlin 1.3.20 intead of 1.3.11 (#322)

Latest gradle prints `The DefaultSourceDirectorySet constructor has been deprecated.` warning.
Upgrading kotlin solves this wanrning.

ref. https://stackoverflow.com/questions/53461821/the-defaultsourcedirectoryset-constructor-has-been-deprecated-how-to-use-the-ob
line/line-bot-sdk-java,line/line-bot-sdk-java,line/line-bot-sdk-java,line/line-bot-sdk-java