chore(gemspec): don't include Gemfile.lock

There's no practical reason to include this in the distributed gem
bundle since applications won't use it.
Including this file can and does cause vulnerability scanners to think
this Gemfile.lock is used by the scanned application.

rubyXL.gemspec

@@ -24,7 +24,6 @@ Gem::Specification.new do |s|
     ".rubocop.yml",
     "CHANGELOG.md",
     "Gemfile",
-    "Gemfile.lock",
     "LICENSE.txt",
     "README.rdoc",
     "Rakefile",