Fix handling of pre-release option.

[zvezdan]

Pex takes a --pre option from the command line, but it does not pass
it correctly during build. We need to propagate it to resolver to get
the correct behavior.

Fixes #423

[zvezdan]

Can someone please re-run the jobs.
Only PyPy and Python 3.4 jobs failed.
Python 3.4 practically did not start, while PyPy job failed in a way that does not seem related to this change.
Both of these jobs have this info at the top of the log:

This job ran on our Precise environment, which is in the process of being decommissioned.
Please read about the status of the rollout on the blog, and take note that you can
explicitly stay on Precise by specifying dist: precise in your .travis.yml.

Since all other language versions are passing I think that re-run or fix of Travis configuration may enable this change to pass the tests completely. Before submitting this PR, I tested with tox and tests for 2.7, 3.6, style, and isort all passed in my local environment. I do not have 3.4 or PyPy to test.

[kwlzn]

lgtm. thanks for the PR.

[kwlzn]

actually, one quick ask: this access of _allow_prereleases violates the private convention. would you mind exposing _allow_prereleases here as a public attribute of the ResolverOptionsBuilder class and using that instead? a quick @property def allow_prereleases(self): return self._allow_prereleases would work.

[zvezdan]

Sure. That's easy. The only reason I did not do it initially is because I did not know what would be the code owner's preferences. I'll do it and resubmit.

[zvezdan]

Actually, I just realized that there was another reason. The existing method:

  def allow_prereleases(self, allowed):
    self._allow_prereleases = allowed
    return self

is not exactly a property setter, because it returns self.
I'll try to work around the existing interface and see what feels the best.

[zvezdan]

@kwlzn I added the change and the comment describing why the property has a different name/API to-do note. If you feel I should take any of the new comments out of the patch, please let me know.
Also, if you feel the name should be different, I'd be glad to adapt to code owner's preferences.
Thanks for the quick review!

[kwlzn]

@zvezdan thanks - merged.