Cannot load such file -- tilt/handlebars

I have an application which has been doing great so far.

However, out of the blue I’m suddenly getting the following error:

.../.rvm/gems/ruby-2.4.0/gems/tilt-2.0.6/lib/tilt/mapping.rb:243:in `require': \
    cannot load such file -- tilt/handlebars (LoadError)

Ruby version:

$ ruby -v
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-linux]

Gemfile:

source "https://rubygems.org"

gem "middleman", "~> 4.2"
gem "middleman-livereload", "~> 3.4"
gem "middleman-protect-emails"
gem "middleman-dotenv", "~> 2.0"
gem "middleman-google-analytics", "~> 2.1"

gem "foreman"
gem "puma"
gem "rake"
gem "slim", "~> 3.0.6"

Gems:

$ gem list | grep middleman
middleman (4.2.1)
middleman-cli (4.2.1)
middleman-core (4.2.1)
middleman-dotenv (2.0.0)
middleman-google-analytics (2.1.1)
middleman-livereload (3.4.6)
middleman-protect-emails (0.4.0)

Any ideas what I might be doing wrong?