[bug] Retrying fetcher due to error (2/4): Bundler::Fetcher::CertificateFailureError

when bundle install errors occur like:

Fetching source index from https://rubygems.org/

Retrying fetcher due to error (2/4): Bundler::Fetcher::CertificateFailureError Could not verify the SSL certificate for https://rubygems.org/.

or

Fetching source index from https://gems.ruby-china.org/

Retrying fetcher due to error (2/4): Bundler::HTTPError Could not fetch specs from https://gems.ruby-china.org/

This can be caused by network failure.
Otherwise we can change alternative sources to fix it.
For thoses in china, modify your GemFile with

source "https://gems.ruby-china.com"