Bump RSpec version.

Alternate fix for https://github.com/codahale/bcrypt-ruby/pull/60 based on feedback from @tmm1
Fix gem source warning from Bundler.

Before:

```
$ bundle install
The source :rubygems is deprecated because HTTP requests are insecure.
Please change your source to 'https://rubygems.org' if possible, or 'http://rubygems.org' if not.
```

After: no warning!
Merge pull request #62 from tjschuck/fix_gemfile_source_warning

Fix gem source warning from Bundler.
Merge pull request #61 from tjschuck/bump_rspec_version_dependency

Bump RSpec version.
Allow a default cost to be set externally

Currently, if you're using bcrypt-ruby externally (like in a Rails app), you can't re-set the default cost to match your own business concerns.

It seems like a common desire to do something like   BCrypt::Engine::DEFAULT_COST = 12 in your config/environments/production.rb, and a lower value in development, and an even lower value in test (although that last case is actually already handled in Rails internals).

This _works_, but causes a "warning: already initialized constant DEFAULT_COST".

This fix allows setting BCrypt::Engine.cost = some_value externally, defaulting internally to DEFAULT_COST.
Merge pull request #63 from tjschuck/make_cost_globally_changeable_externally

Allow a default cost to be set externally
Custom getter/setter for BCrypt::Engine.cost

To maintain backwards compatibility for people who were manually overriding the BCrypt::Engine::DEFAULT_COST externally.

Via @tmm1 on https://github.com/codahale/bcrypt-ruby/pull/63
Merge pull request #64 from tjschuck/make_cost_globally_changeable_externally

Custom getter/setter for BCrypt::Engine.cost