Could not find thread_safe-0.3.0 in any of the sources

I was experiencing the same problem.

The thread_safe gem version 0.3.0 has been yanked now from Ruby Gems: http://rubygems.org/gems/thread_safe/versions

So you might want to specify a different version in your Gemfile, or you can point the Gemfile directly to GitHub.

I solved the problem with this line in my Gemfile:

gem 'thread_safe', '0.2.0'

As dreamnid said, "Look like they yank the 0.3.0 build a few hours ago to troubleshoot a build issue".

Try to run gem uninstall thread_safe (to remove yanked version) and later bundle update to install the correct versions.

Running:

bundle update

Fixed the issue for me, with the message: "Using thread_safe 0.2.0 (was 0.3.0)"