(#15464) Exclude gem command from bundler

This patch simply excludes the `gem` command from the dependency
handling of Bundler with the `rubygems-bundler` gem installed.

This patch also updates the README to describe the resolution to the
spec test failures we've been seeing when Puppet contains a Gemfile in
the project root.

To summarize: Use of RVM is considered dangerous

We're able to work around the problem, but the hijacking of shell
builtin commands is a maintenance burden we're not willing to shoulder,
hence RVM being considered a dangerous tool.

.noexec.yaml

@@ -1,4 +1,5 @@
 --- 
 exclude:
+ - gem
  - rake
  - rspec

README_DEVELOPER.md

@@ -3,6 +3,28 @@
 This file is intended to provide a place for developers and contributors to
 document what other developers need to know about changes made to Puppet.
 
+# Use of RVM considered dangerous #
+
+Use of RVM in production situations, e.g. running CI tests against this
+repository, is considered dangerous.  The reason we consider RVM to be
+dangerous is because the default behavior of RVM is to hijack the builtin
+behavior of the shell, causing Gemfile files to be loaded and evaluated when
+the shell changes directories into the project root.
+
+This behavior causes the CI Job execution environment that runs with `set -e`
+to be incompatible with RVM.
+
+We work around this issue by disabling the per-project RC file parsing using
+
+    if ! grep -qx rvm_project_rvmrc=0 ~/.rvmrc; then
+      echo rvm_project_rvmrc=0 >> ~/.rvmrc
+    fi
+
+When we setup CI nodes, but this is not standard or expected behavior.
+
+Please consider rbenv instead of rvm.  The default behavior of rvm is difficult
+to maintain with `set -e` shell environments.
+
 # Dependencies #
 
 Puppet is considered an Application as it relates to the recommendation of