Upgrade Bundler from 1.17.2 to 2.1.4
We need to explicitly install Bundler 2.1.4 since we are using Ruby 2.5 package from Debian buster, which does not bundle Bundler 2.x.
Signed-off-by: Takuya Noguchi takninnovationresearch@gmail.com
Closes #483 (closed)
Edited by Takuya Noguchi