Temporarily rename the .ruby-version file to pass CI for rubocop-old

Old versions of rubocop read the ruby version from .ruby-version first.
We made the change upstream which is not captured in versions of
rubocop (< 0.87).

Upstream commit: rubocop/rubocop@02b2c3a

spec/erb_lint/linters/rubocop_spec.rb

@@ -30,6 +30,8 @@
     )
   end
   before { linter.run(processed_source) }
+  before(:all) { File.rename(".ruby-version", ".ruby-version.bak") }
+  after(:all) { File.rename(".ruby-version.bak", ".ruby-version") }
 
   context "config is valid when rubocop_config is not explicitly provided" do
     let(:linter_config) do