TST: adding sdist and install tests to the 2.6 regression - closes #9878
closes #9878 

This PR adds a test for packaging and installing pandas from a tarball, inside of the 2.6 regression.

After regular nosetests complete, cython is removed from the venv, pandas is packaged with cdist command, and the package is installed via pip. Limited nosetests run once installed.

The travis log output is a little messy with the pip command output; ideally I wanted to 'fold'/collapse the output unless there was a failure but I don't think you can do that from within the 'script' portion of the regression, only the before and after scripts, see:
http://blog.travis-ci.com/2013-05-22-improving-build-visibility-log-folds/
If there is a way to do this maybe we can add it later, or move where this INSTALL_TEST is happening to a before or after script.

I _think_ that you can add another entry after the script (and put your change in), maybe something like. `install_test.sh` which will 'fold' it (I think if you run separate scripts it folds that output

This is in `.travis.yml`

```
script:
  - echo "script"
  - ci/script.sh
# nothing here, or failed tests won't fail travis
```
 @jrebeck I gave this a try; it looks like log output will only fold if it is outside the 'script' section of the travis job (here is the log output a separate install_test.sh script: https://travis-ci.org/ajamian/pandas/jobs/58972085)

Is it ok if we put the install_test in say the after_scripts section? I think this would give the folding.
 yep that's ok too
 @jreback looks like folding works for stuff in the after_script section of travis - i put it before everything else in that section.
