Use bundle install --standalone

Create a gem project

$ bundle gem awesome-cmd -b
      create  awesome-cmd/Gemfile
      create  awesome-cmd/Rakefile
      create  awesome-cmd/LICENSE
      create  awesome-cmd/README.md
      create  awesome-cmd/.gitignore
      create  awesome-cmd/awesome-cmd.gemspec
      create  awesome-cmd/lib/awesome-cmd.rb
      create  awesome-cmd/lib/awesome-cmd/version.rb
      create  awesome-cmd/bin/awesome-cmd
Initializating git repo in /path/to/awesome-cmd

Run bundle install standalone

$ cd awesome-cmd
$ vi awesome-cmd.gemspec # add gem e.g. 'thor'
$ bundle install --standalone 
Fetching gem metadata from https://rubygems.org/..
Installing thor (0.15.4) 
Using awesome-cmd (0.0.1) from source at /path/to/awesome-cmd 
Using bundler (1.1.4) 
Your bundle is complete! It was installed into ./bundle

Use ruby instead of bundle exec

$ ruby -r ./bundle/bundler/setup.rb ./bin/awesome-cmd --version
0.0.1