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

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 output of this test in the log is now 'folded' via http://blog.travis-ci.com/2013-05-22-improving-build-visibility-log-folds/ by putting install_test.sh into the 'after_script' section of the travis config.

This is a new PR with squashed commits to replace the original closed PR: https://github.com/pydata/pandas/pull/9922

@ajamian awesome thanks!
