Fix RUBYOPT usages

There's a warning regarding an unknown argument to `--disable`, and it
seems that `--disable=gem` is not entirely correct, should be
`--disable=gems`
[Documentation](https://docs.ruby-lang.org/ja/latest/doc/spec=2frubycmd.html)
(in Japanese).

Example output if you pass something that's completely off:

    ruby: warning: unknown argument for --disable: `jam'
    ruby: warning: features are [gems, did_you_mean, rubyopt, frozen_string_literal].

Author: pirj

example_app_generator/travis_retry_bundle_install.sh

@@ -5,7 +5,7 @@ source FUNCTIONS_SCRIPT_FILE
 
 echo "Starting bundle install using shared bundle path"
 if is_mri_192_plus; then
-  travis_retry eval "RUBYOPT=$RUBYOPT:' --enable rubygems' bundle install --gemfile ./Gemfile --path REPLACE_BUNDLE_PATH --retry=3 --jobs=3"
+  travis_retry eval "RUBYOPT=$RUBYOPT:' --enable gems' bundle install --gemfile ./Gemfile --path REPLACE_BUNDLE_PATH --retry=3 --jobs=3"
 else
   travis_retry eval "bundle install --gemfile ./Gemfile --path REPLACE_BUNDLE_PATH --retry=3 --jobs=3"
 fi

script/functions.sh

@@ -14,7 +14,7 @@ MAINTENANCE_BRANCH=`cat maintenance-branch`
 # Don't allow rubygems to pollute what's loaded. Also, things boot faster
 # without the extra load time of rubygems. Only works on MRI Ruby 1.9+
 if is_mri_192_plus; then
-  export RUBYOPT="--disable=gem"
+  export RUBYOPT="--disable=gems"
 fi
 
 function clone_repo {