dev: reload_test_env(): run all rake tasks in a single bundle exec

This is faster and also matches how Travis does it (almost).
Merge pull request #1751 from aspiers/topic/speed-up-test-reload

dev: reload_test_env(): run all rake tasks in a single bundle exec
Merge pull request #1754 from rhafer/master

Make sure gemfile_plugin is actually synced to disk before rake task are executed
File::new() does not take block; use File::open() instead

Hotfix for breakage introduced by e3efe5ba :(