Lock turbo_tests to 2.1.0 provisionally

[nobu]

turbo_tests 2.1.1 adds json to its dependency and the current bundler does not take the standard library json and fails to build as a gem before the installation.

[ilyazub]

I'm sorry for introducing this issue with turbo_tests. We'll remove the json dependency.

Just documenting the commands/pristine_spec.rb failure to be able to verify verify the fix later.

$ make test-bundler-parallel BUNDLER_SPECS=commands/pristine_spec.rb
# ...omitted
Invoking `/omitted/path/ruby/build/ruby -I/omitted/path/ruby/spec/bundler -r/omitted/path/ruby/spec/bundler/support/artifice/fail.rb -r/omitted/path/ruby/spec/bundler/support/hax.rb /omitted/path/ruby/tmp/1/gems/system/bin/bundle pristine` failed with output:
----------------------------------------------------------------------
Cannot pristine bar (1.0). Gem is sourced from local path.
Cannot pristine baz (1.0.0). Gem is sourced from local path.
The installation path is insecure. Bundler cannot continue.
/omitted/path/ruby/tmp/1/gems/system/extensions/x86_64-linux/3.4.0+0-static is world-writable (without sticky bit).
Bundler cannot safely replace gems in world-writeable directories due to potential vulnerabilities.
Please change the permissions of this directory or choose a different install path.
Installing very_simple_binary 1.0 with native extensions
Installing baz-dev 1.0.0
Installing weakling 1.0
----------------------------------------------------------------------
# ./spec/bundler/support/helpers.rb:208:in 'Spec::Helpers#sys_exec'
# ./spec/bundler/support/helpers.rb:121:in 'Spec::Helpers#bundle'
# ./spec/bundler/commands/pristine_spec.rb:122:in 'block (3 levels) in <top (required)>'
# ./spec/bundler/spec_helper.rb:106:in 'block (4 levels) in <top (required)>'
# ./spec/bundler/spec_helper.rb:106:in 'block (3 levels) in <top (required)>'
# ./spec/bundler/support/helpers.rb:339:in 'block in Spec::Helpers#with_gem_path_as'
# ./spec/bundler/support/helpers.rb:353:in 'Spec::Helpers#without_env_side_effects'
# ./spec/bundler/support/helpers.rb:334:in 'Spec::Helpers#with_gem_path_as'
# ./spec/bundler/spec_helper.rb:105:in 'block (2 levels) in <top (required)>'