Fix test case failures on JRuby >= 1.7.5.

- DateTime on JRuby 1.7.5 only allows offsets of less than 1 day. Change
  uses of 1 day offsets to 1 hour.
- JRuby 1.7.5 considers DateTime instances that differ by less than 1
  millisecond to be equivalent
  (https://github.com/jruby/jruby/issues/1311). Make tests that rely on
  being able to distinguish differences of 1 microsecond adapt if the
  platform doesn't support such comparisons.
Update test environments.

- Add ruby-2.1.0-preview2.
- Add rbx-2.
- Remove no longer supported rbx-18mode and rbx-19mode.