add bundler as a dependency

[tongueroo]

This is a 🐞 bug fix.

• I've added tests (if it's a bug, feature or enhancement)
• I've adjusted the documentation (if it's a feature or enhancement)
• The test suite passes (run bundle exec rspec to verify this)

Summary

Bundler should be a dependency and installed a part of terraspace.

Context

Ran into this while looking into:

• terraspace build failing - `spawn': No such file or directory - file (Errno::ENOENT)  #163
• check if file command is installed #165

How to Test

Sanity check of terraspace by trying out the quick start example on a fresh machine. IE: docker or cloud9, etc

Version Changes

Patch