No need to load mutant automatically with Bundler

* mutant is run through its commandlne exe
* normally we do not require mutant files in specs
* there's rare case when invoking Rails::Generator somehow triggers
  requiring mutant gem, perhaps via some obscure code in Rails calling
  Bundler.require underneath

Resolves problems with truffleruby:
oracle/truffleruby#2798

support/bundler/Gemfile.shared

@@ -1,6 +1,6 @@
 gem "rake", ">= 10.0"
 gem "rspec", "~> 3.6"
-gem "mutant-rspec", "= 0.11.17"
-gem "mutant-minitest", "= 0.11.17"
-gem "mutant", "= 0.11.17"
+gem "mutant-rspec", "= 0.11.17",    require: false
+gem "mutant-minitest", "= 0.11.17", require: false
+gem "mutant", "= 0.11.17",          require: false
 gem "mutant-license", source: "https://oss:7AXfeZdAfCqL1PvHm2nvDJO6Zd9UW8IK@gem.mutant.dev"