migrate to using miniconda

Anaconda is great though I suggest that the closely related Miniconda is even more on target for Heroku deployments because it seeks to install _only_ the packages required (whereas Anaconda has a larger core set of packages that come as part of any install, before further customization).

The proposed changes in this pull request provide:
* smaller slug sizes (a show-stopping issue when the slug size is too large for Heroku's 300Mb limit),
* still use conda-requirements.txt file in the same way to control binary package installs,
* permits processing of requirements.txt via pip *after* conda has installed its packages for anything not easily installed via conda (but still available via pip),
* by pointing at http://repo.continuum.io/miniconda/Miniconda-latest-Linux-x86_64.sh this buildpack is kept up-to-date over time without impact to the versions specified in various requirements.txt files,
* addresses Open Issue 1 (https://github.com/kennethreitz/anaconda-buildpack/issues/1) by keeping separate controls for conda and pip while supporting both because the format required by `conda --file` is distinct from the format required by `pip -r` and so the creation of conda-requirements.txt should really be managed via conda itself (via `conda list -e` in the user's current working conda environment),
* addresses Open Issue 3 (https://github.com/kennethreitz/anaconda-buildpack/issues/3) due to changes in how conda is invoked here as well as changes in conda over the last few months,
* enables installation of combinations of scipy+numpy that have been problematic via pip,
* similarly enables installation of scikit-image (which was not possible for me simply via a single pip requirements.txt pass).

I realize you likely haven't touched this project in some time.  There are still other aspects of the scripts in this buildpack that deserve further investigation but I believe the changes I'm proposing already result in a generally useful/reusable package.  Thank you for having taken the time to put together this buildpack in the first place -- without your efforts I would never have been inspired to make this work.