cocoapods: fix deprecation about Gemfile

> [DEPRECATED] This Gemfile does not include an explicit global source. Not using an explicit global source may result in a different lockfile being generated depending on the gems you have installed locally before bundler is run. Instead, define a global source in your Gemfile like this: source "https://rubygems.org".

pkgs/development/tools/cocoapods/Gemfile

···
-source 'https://rubygems.org' do
-  gem 'cocoapods'
-end
+source 'https://rubygems.org'
+
+gem 'cocoapods'

pkgs/development/tools/cocoapods/Gemfile-beta

···
-source 'https://rubygems.org' do
-  # We need to specify a version including prerelease to pick up prereleases.
-  # This should pick up all future releases / prereleases.
-  gem 'cocoapods', '>=1.7.0.beta.1'
-end
+source 'https://rubygems.org'
+
+# We need to specify a version including prerelease to pick up prereleases.
+# This should pick up all future releases / prereleases.
+gem 'cocoapods', '>=1.7.0.beta.1'

pkgs/development/tools/cocoapods/Gemfile-beta.lock

···
-GEM
-  specs:
-
 GEM
   remote: https://rubygems.org/
   specs:
···
   ruby
 
 DEPENDENCIES
-  cocoapods (>= 1.7.0.beta.1)!
+  cocoapods (>= 1.7.0.beta.1)
 
 BUNDLED WITH
    2.2.20

pkgs/development/tools/cocoapods/Gemfile.lock

···
-GEM
-  specs:
-
 GEM
   remote: https://rubygems.org/
   specs:
···
   ruby
 
 DEPENDENCIES
-  cocoapods!
+  cocoapods
 
 BUNDLED WITH
    2.2.20