Temp pin rspec-core

Author: JonRowe

Gemfile

@@ -4,14 +4,15 @@ source 'https://rubygems.org'
 gemspec
 
 branch = File.read(File.expand_path("../maintenance-branch", __FILE__)).chomp
-%w[rspec rspec-core rspec-expectations rspec-mocks].each do |lib|
+%w[rspec rspec-expectations rspec-mocks].each do |lib|
   library_path = File.expand_path("../../#{lib}", __FILE__)
   if File.exist?(library_path) && !ENV['USE_GIT_REPOS']
     gem lib, :path => library_path
   else
     gem lib, :git => "https://github.com/rspec/#{lib}.git", :branch => branch
   end
 end
+gem "rspec-core", :git => "https://github.com/rspec/rspec-core.git", :branch => "update-travis-build-scripts-2019-12-26-for-master"
 
 ### dep for ci/coverage
 gem 'simplecov', '~> 0.8'