Improve testing

Suite changes

* Use minitest 5
* Implement test runner as rake task, remove test_all.sh
* Extract common code to helper
* Keep and run isolated tests separately
* Nest classes used in tests inside tests
* Use minitest-around to ensure default options are reset
* Get rid from warnings (unused variables)
* Keep all temporary files in test directory
* Remove unnecessary files from repo, improve gitignore
* Enable testing on Travis

Test changes

* Add rails integration tests
* Remove 'rails like' test
* Improve mimic before/after tests
* Add mimic_single in order to properly test in isolation
* Remove unimplemented test from test_debian
Improve testing: fix some tests, get 1.8.7 and rbx working
Improve testing: allow 1.8.7 failures, do no run test_mimic_rails for now
Improve testing: bring back test_all.sh
Merge pull request #153 from krasnoukhov/improve-testing

Improve testing
Improve mimic of JSON::ParserError

* Do const_set on Ruby level instead of C
* Extract common mimic tests to shared_mimic
* Make sure before/after tests are working
* Improve mimic_single test

As a result of this Rails integration is working properly.
The only caveeat is that Oj.mimic_JSON should be called after
json is loaded for this case. Basically this means that
it should be called after Rails is loaded.
Improve mimic: bring back C const assignment
Improve mimic: add test_activesupport_encode test
Merge branch 'mimic-rails'