File.exists? has been deprecated since Ruby 1.9.2

Fixes #1550

* `fog_spec.rb:328` fails intermittently due to a race condition
* Use [TimeCop][1] gem to freeze time during the spec
* Extract magic number
* Break long line

[1]: https://github.com/travisjeffery/timecop

![TimeCop
Poster](http://www.deconstructingmovies.com/wp-content/uploads/2014/03/timecop-poster.jpg)

There's nothing they can be spoofing, and there's little danger of a
webserver accidentally sending a dangerous Content-Type with a file that
has no extension. If the browser chooses to interpret it in a dangerous
manner, no server-side library on earth can change that.

This isn't specifically related to RSpec, but is built off the branch
and is part of an effort to get the CI build green.

First off, sqlite3 needs to be exactly v1.3.8.
Need to make sure both racc and rubysl are in the Gemfiles.
RBX's File.join doesn't like nil at all.
RBX also doesn't like to remove a constant that doesn't exist yet.

There were a bunch of free-floating methods in spec_helper. They were
pulled over from test/helper, but they shouldn't be there. Move them
into appropriate modules in spec/support. This commit also cleans up the
"should_accept/reject_dummy_class methods in favor of using proper
matchers directly. This commit should probably not get squashed or git
will likely forget that those matcher specs are the same file because of
how much changed.

This is a big one.

This change converts all of the existing tests from test/unit to RSpec.
It's been a long time coming, and was specifically prompted by the
inability of minitest to work with Appraisal across the various Rails
versions. Instead of ironing out that headache, converting to RSpec was
preferable (after initial probing).

Many assertions are still assertions and are housed in the Assertions
module, but they should be converted as they get touched. Similarly,
since we're touching all the tests/specs, we convert all the hashes from
1.8 syntax to 1.9 syntax.