travis: mac: adjust bundler uninstall command

Author: efiop

scripts/build_posix.sh

@@ -89,7 +89,7 @@ install_dependencies()
 	gem install --no-document fpm
 
 	# https://github.com/iterative/dvc/issues/2970
-	gem uninstall rubygems-bundler
+	gem uninstall -i /Users/travis/.rvm/gems/ruby-2.4.3@global rubygems-bundler
 
 	print_info "Upgrading pip..."
 	pip install --upgrade pip