Jekyll site deployment to Netlify - Error gem bundle install (use Bundler 2 or >) / Failing build - github

Question

I'm using Cloud9 and GitHub (Private Repos). I've been able to deploy basic landing pages to Netlify but I keep getting the same error message when I try to deploy any Jekyll sites. Snippet from deployment log:
> Installing gem bundle
> You must use Bundler 2 or greater with this lockfile.
> Error during gem install
> failed during stage 'building site': Build script returned non-zero exit code: 1
> Error running command: Build script returned non-zero exit code: 1
> Failing build: Failed to build site
After many many failed attempts, I created a new test dev environment. Per Jekyll docs I made sure I had the latest Ruby, RubyGems, gcc, g++, make & dependencies. Did the usual config, then Jekyll new test_directory. Steps:
> gem install jekyll bundler
> gem install bundle
> gem update --system
> cd new_directory && bundle install
> ~/new_directory/ bundle update
> ~/new_directory/ bundle exec jekyll serve
*** Works perfectly on localhost ***
At this point everything looks fine (jekyll 3.8.5 - gem 3.0.2 - ruby 2.5.1p57 (2018-03-29 revision 63029) [x86_64-linux])
Important:
> bundler -v
returns
> Bundler version 2.0.1
After I push to completely new repo, I get the same error message as above:
"use Bundler 2 or greater with this lockfile"
Snippet from lockfile:
> DEPENDENCIES
> -jekyll (~> 3.8.5)
> -jekyll-feed (~> 0.6)
> -minima (~> 2.0)
> -tzinfo-data
> RUBY VERSION
> ruby 2.5.1p57
> BUNDLED WITH
> 2.0.1
I also added this to _config.yml file:
> url: "https://infallible-hodgkin-e15692.netlify.com/"
And per Netlify docs I added a /.ruby-version file inside my root directory with this line:
> ruby 2.5.1
After updating everything and git push, it still fails to build my site but this time I get a couple new lines on the deploy log:
>** WARNING **
> Using custom ruby version ruby 2.5.1, this will slow down the build.
> To ensure fast builds, set the RUBY_VERSION environment variable, or .ruby-version file, to an included ruby version.
> Included versions: 2.3.6 2.4.3 2.2.9
Needless to say I tried switching to each "Included" Ruby -v (rvm use ..*). Went through the process of installing each version and repeated steps above each time. No luck.
Any suggestions as to what to try next are greatly appreciated. Sorry for the long post, I tried being as concise as possible. Please let me know if I missed anything.

Answer 1

So the problem was I should've been using Bundle 1 instead of Bundle 2 plus ruby 2.3.6. Make sure you're on bundle version 1. You can read a little more here:
https://bundler.io/guides/bundler_2_upgrade.html
switch to ruby 2.3.6:
> rvm use 2.3.6 (it'll give you the prompt to install it)
run this command on your project's root directory check if it worked by running bundle version
> gem install bundler -v "~>1.0"
then
> gem install jekyll
> gem install bundle
Here's what I changed on mi Gemfile.lock:
> RUBY VERSION
> ruby 2.3.6p384
> BUNDLED WITH
> 1.17.3
My Gemfile just says ruby "2.3.6" at the top and I also change the /.ruby-version file to ruby 2.3.6 (no quotes). And that's it. Don't forget to bundle exec jekyll serve, commit and push and it should be fine.