Migrate to SpotBugs from FindBugs.
Migrate to SpotBugs from FindBugs.

FindBugs no longer maintenanced and SpotBugs is a successor.
See https://mailman.cs.umd.edu/pipermail/findbugs-discuss/2017-September/004383.html

This closes https://github.com/line/line-bot-sdk-java/issues/215

line/line-bot-sdk-java,c963951/briantest,line/line-bot-sdk-java,line/line-bot-sdk-java,line/line-bot-sdk-java