Avoid prefixing buildout dir to 'special' rpath elements.

Noted the change in buildout's changelog, too, as that's much more prominent
and as it fixes a buildout bug.

Travis speed improvement through caching

(Good test to see if it improved travis' speed!)

The cache mechanism of travis-ci.org itself creates the cache directory, so it
is already there before we try to add it.
The bin/python inside it seems a good target.

Disabling sudo enables the use of the container based infra, necessary for
caching.
The directory with the downloaded and compiled python is cached.

Use local ez_setup.py if one exists; add --setuptools-download-base option

The pull request itself contains a change that should not be applied.

Run easy_install with specified setuptools_version, same change as #232 

Run easy_install with specified setuptools_version

Use 'rcN' for release candidate versions in tests

Our VersionConflict handling could result in an UnpackError when rendering

Potential fix for https://github.com/buildout/buildout/issues/217

If setuptools is installed globally by easy_install then the easy_install.pth gets added to sys.path before PYTHONPATH and the wrong setuptools version is used.

Recent setuptools versions changed the canonical spelling of release
candidate versions from 'cN' to 'rcN'.  Using the older spelling causes
a UserWarning about the version requiring normalization, and test
failures where the expected output contains a different (normalized)
version number.

For the record, the UserWarning looks like this:

/home/mg/src/buildout/eggs/setuptools-11.3.1-py2.7.egg/setuptools/dist.py:283:
UserWarning: The version specified requires normalization, consider
using '1.2rc1' instead of '1.2c1'.

and the failures can be seen at
https://travis-ci.org/buildout/buildout/jobs/46215472

Don't assume pkg_resources is a module.