Use latest gradle (v6.1) with jruby-gradle v1.5.0

[civitaspo]

This plugin uses the very old gradle, so I changed the version to the latest one. And I change the jruby-gradle dependency because jruby-gradle v0.1.5 already cannot release gems.

I will be making a lot of Pull Requests for the next release, so please don't release this change as a new version yet.

[civitaspo]

The tests were broken before I made this change. I will fix it with another pull request later. (EDIT: I had the wrong way to run the test.)

[civitaspo]

@lulichn How can I run the tests? This seems to work when I set the environment variables appropriately, but I cannot know the variables... ref. https://circleci.com/gh/lulichn/embulk-input-dynamodb/49

[civitaspo]

@lulichn How can I run the tests? This seems to work when I set the environment variables appropriately, but I cannot know the variables... ref. https://circleci.com/gh/lulichn/embulk-input-dynamodb/49

I got it. These variables are generated in your AWS account, right? The tests are succeeded when I create IAM User which have the AmazonDynamoDBFullAccess policy and I generate credentials and test data.

So, this pull-request is ready to merge.