Can't cofigure gitlab-development-kit: gitlab_development_kit/lib/runit/config.rb:100:in `delete_exec_prefix!': undefined method `delete_prefix!'

Expected outcome: The gdk start command runs succesfully after following the Setup GDK instructions on a MacOS machine configured based on the Prepare instructions.

Actual outcome: The gdk start command fails due to an undefined method

Steps to reproduce:

$ ruby -v
ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-darwin18]

$ gem install gitlab-development-kit
Fetching gitlab-development-kit-0.2.5.gem
Successfully installed gitlab-development-kit-0.2.5
Parsing documentation for gitlab-development-kit-0.2.5
Installing ri documentation for gitlab-development-kit-0.2.5
Done installing documentation for gitlab-development-kit after 0 seconds

$ gdk init && cd gitlab-development-kit
...

$ gdk install gitlab_repo=git@gitlab.com:jamestharpe/gitlab.git
...

$ support/set-gitlab-upstream
Remote origin is gitlab, adding remote upstream to gitlab
Fetching master from upstream...
remote: Enumerating objects: 17004, done.
remote: Counting objects: 100% (17004/17004), done.
remote: Compressing objects: 100% (8101/8101), done.
remote: Total 17004 (delta 11108), reused 13423 (delta 8489), pack-reused 0
Receiving objects: 100% (17004/17004), 10.15 MiB | 2.31 MiB/s, done.
Resolving deltas: 100% (11108/11108), completed with 1289 local objects.
From https://gitlab.com/gitlab-org/gitlab
 * branch                    master     -> FETCH_HEAD
 * [new branch]              master     -> upstream/master
Branch 'master' set up to track remote branch 'master' from 'upstream'.

$ gdk start                                                                                                                                          git:(master|) 
(in /Users/jtharpe1/code/gitlab-development-kit)
/Users/jtharpe1/code/gitlab-development-kit/lib/runit/config.rb:100:in `delete_exec_prefix!': undefined method `delete_prefix!' for #<String:0x00007fbd3c015940> (NoMethodError)
	from /Users/jtharpe1/code/gitlab-development-kit/lib/runit/config.rb:61:in `block in render'
	from /Users/jtharpe1/code/gitlab-development-kit/lib/runit/config.rb:54:in `map'
	from /Users/jtharpe1/code/gitlab-development-kit/lib/runit/config.rb:54:in `render'
	from /Users/jtharpe1/code/gitlab-development-kit/lib/runit.rb:23:in `start_runsvdir'
	from /Users/jtharpe1/code/gitlab-development-kit/lib/runit.rb:139:in `sv'
	from /Users/jtharpe1/code/gitlab-development-kit/lib/gdk.rb:79:in `main'
	from /Library/Ruby/Gems/2.3.0/gems/gitlab-development-kit-0.2.5/bin/gdk:74:in `launcher_main'
	from /Library/Ruby/Gems/2.3.0/gems/gitlab-development-kit-0.2.5/bin/gdk:125:in `<top (required)>'
	from /usr/local/bin/gdk:22:in `load'
	from /usr/local/bin/gdk:22:in `<main>'