vagrant: switch from basebox to generated box
bootstrap: update virtualenv to prevent symlinking with --always-copy
bootstrap: add 1.5Mb pip + setuptools for virtualenv to work

Otherwise it fails with:
...
==> gratipay: Installing setuptools, pip...
==> gratipay:   Complete output from command /vagrant/env/bin/python2.7 -c "import sys, pip; sys...d\"] + sys.argv[1:]))" setuptools pip:
==> gratipay:   Traceback (most recent call last):
==> gratipay:   File "<string>", line 1, in <module>
==> gratipay: ImportError: No module named pip
==> gratipay: ----------------------------------------
==> gratipay: ...Installing setuptools, pip...done.
==> gratipay: Traceback (most recent call last):
==> gratipay:   File "./vendor/virtualenv-1.11.6.py", line 2338, in <module>
==> gratipay:
==> gratipay: main()
==> gratipay:   File "./vendor/virtualenv-1.11.6.py", line 824, in main
==> gratipay:
==> gratipay: symlink=options.symlink)
==> gratipay:   File "./vendor/virtualenv-1.11.6.py", line 992, in create_environment
==> gratipay:
==> gratipay: install_wheel(to_install, py_executable, search_dirs)
==> gratipay:   File "./vendor/virtualenv-1.11.6.py", line 960, in install_wheel
==> gratipay:
==> gratipay: 'PIP_NO_INDEX': '1'
==> gratipay:   File "./vendor/virtualenv-1.11.6.py", line 902, in call_subprocess
==> gratipay:     % (cmd_desc, proc.returncode))
==> gratipay: OSError: Command /vagrant/env/bin/python2.7 -c "import sys, pip; sys...d\"] + sys.argv[1:]))" setuptools pip failed with error code 1
==> gratipay: make:
==> gratipay: *** [env] Error 1