[#789] Fix the "explain" tests to conform to the new response format
[#789] Fix the mvel expression in `custom_score` query test
[#789] Use the `:refresh` option in document update tests
[#789] Only run percolator tests when the `TRAVIS` environment variable is set

This should be set by Travis automatically [http://about.travis-ci.org/docs/user/ci-environment/#Environment-variables],
to run the tests locally, just set the variable:

    $ TRAVIS=true time bundle exec ruby -I lib:test test/integration/percolator_test.rb