[SETUP] Update Travis-CI configuration (remove REE, restructure)
[SETUP] Use only "shoulda-context", not "shoulda"

The "shoulda" gem brings in the "shoulda-matchers" gem, which is not neccessary,
slows down the require time, and fails on 1.8.7:

    Gem::InstallError: shoulda-matchers requires Ruby version >= 1.9.2.

See <https://travis-ci.org/karmi/tire/jobs/7104954>
[TEST] Delete the whole percolator index on Travis

Still failing... <https://travis-ci.org/karmi/tire/builds/7104949>

Related: #541