Travis tries to install alpha DPL even with custom DPL specified

[maiquelcraash]

Problem

When running the deploy stage, Travis tries to install the alpha version of DPL (note the --pre param in the command below) which is not compatible with Ruby v2.7.

$ rvm use $(travis_internal_ruby) --fuzzy do ruby -S gem install dpl --pre
ERROR:  Error installing dpl:
	The last version of dpl (>= 0) to support your Ruby & RubyGems was 2.0.3.beta.4. Try installing it with `gem install dpl -v 2.0.3.beta.4`
	dpl requires Ruby version >= 3.1. The current ruby version is 2.7.6.219.

I'm defining a specific version of DPL, but Travis is not installing the one I want from My-custom-Repo/dpl

deploy:
  - provider: s3
    ...
    edge:
      source: 'My-custom-Repo/dpl'
      branch: 'master'

I'm also defining the build to use Ruby v2.7 which is the one compatible with my DPL.

before_install:
  - rvm use 2.7.6 --default

Why do I need a custom DPL?
Because of another issue that was never solved #1218

Expected behavior

Travis should not install alpha versions of DPL if a custom version is specified.