How to fix "Your Ruby version is 1.9.3, but your Gemfile specified 2.0.0"

None of these worked for me, but I finally figured it out:

spring stop

I had a Rails 4 project that uses something called Spring to keep a Rails environment running in the background, for speed.

I had started Spring with Ruby 2.1.1 but then I upgraded to 2.1.2 (in Gemfile and .ruby-version). My theory is that Spring kept on running on 2.1.1 and so caused this error.

I've noticed this happening when I've tried to bundle install with a fresh RVM gemset that didn't yet include bundler (as it fell back on the system bundler install which referenced the system ruby).

Running gem install bundler fixed for me.