Instead of always bootstrapping to eggs, only do this in dev mode.

Since egg bootstrapping is just there to support IDEs - implying
PANTS_DEV mode makes sense to be using, and, more importantly,
since travis-ci runs choke on the generated eggs almost 100% of the
time [1], just use eggs in dev mode.

[1] pip fails like so:
  Running setup.py install for twitter.common.dirutil

    installing library code to build/bdist.linux-x86_64/egg
    removing 'build/bdist.linux-x86_64/egg' (and everything under it)
    Processing twitter.common.dirutil-0.1.3-py2.7.egg
    Removing /home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/lib/python2.7/site-packages/twitter.common.dirutil-0.1.3-py2.7.egg
    twitter.common.dirutil 0.1.3 is already the active version in easy-install.pth

    Installed /home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/lib/python2.7/site-packages/twitter.common.dirutil-0.1.3-py2.7.egg
    Processing dependencies for twitter.common.dirutil==0.1.3
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/build/twitter.common.dirutil/setup.py", line 22, in <module>
        'zip_safe': True}
      File "/usr/lib/python2.7/distutils/core.py", line 152, in setup
        dist.run_commands()
      File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
        self.run_command(cmd)
      File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
        cmd_obj.run()
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/setuptools/command/install.py", line 74, in run
        self.do_egg_install()
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/setuptools/command/install.py", line 97, in do_egg_install
        cmd.run()
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 358, in run
        self.easy_install(spec, not self.no_deps)
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 574, in easy_install
        return self.install_item(None, spec, tmpdir, deps, True)
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 625, in install_item
        self.process_distribution(spec, dist, deps)
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/setuptools/command/easy_install.py", line 671, in process_distribution
        [requirement], self.local_index, self.easy_install
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 593, in resolve
        requirements.extend(dist.requires(req.extras)[::-1])
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 2251, in requires
        dm = self._dep_map
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 2237, in _dep_map
        for extra,reqs in split_sections(self._get_metadata(name)):
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 2673, in split_sections
        for line in yield_lines(s):
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 1949, in yield_lines
        for ss in strs:
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 2265, in _get_metadata
        for line in self.get_metadata_lines(name):
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 1329, in get_metadata_lines
        return yield_lines(self.get_metadata(name))
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 1321, in get_metadata
        return self._get(self._fn(self.egg_info,name))
      File "/home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/local/lib/python2.7/site-packages/pkg_resources.py", line 1385, in _get
        return self.loader.get_data(path)
    zipimport.ZipImportError: bad local file header in /home/travis/build/pantsbuild/pants/build-support/pants_deps.venv/lib/python2.7/site-packages/twitter.common.dirutil-0.1.3-py2.7.egg

https://rbcommons.com/s/twitter/r/272/