Avoid rubygems loading issue with Bundler 2.1.0

On Bundler 2.1.0 the loading of rubygems has been rewritten. It breaks
our existing usage of command execution. The --disable=gem flag is
problematic because we need to load rubygems in Bundler. It is
not explicitly loaded in Bundle 2.1.0

Related:
  - rubygems/bundler#7487

Author: benoittgt

script/custom_build_functions.sh

@@ -1,6 +1,6 @@
 function run_cukes {
   if is_mri_192_plus; then
-    bin/rake acceptance --trace
+    eval "(unset RUBYOPT; exec bin/rake acceptance --trace)"
     return $?
   elif is_jruby; then
     bin/rake smoke:app