Revise before(:all)/let behavior based on feedback from @dchelimsky.

- This is closer to the behavior we've had in 2.12 and earlier. `let`
  declarations that are accessed in `before(:all)` will preserve their
  memoized value for all examples in the group.
- On 2.12 and earlier, this would cause additional `let` declarations
  that were not accessed in `before(:all)` but were accessed in individual
  examples to "leak", because the same `__@memoized` ivar got shared
  with each example in the group, and would be mutated each time an
  additional `let` declaration was accessed. This commit changes
  that behavior. The fact that one `let` declaration is accessed in
  `before(:all)` does not affect the lifecycle of other `let` declarations
  in any way.
Add changelog entries for #823, #821, #813, #811, and #802.

[ci skip]
Fix README example spec to display actual output
Merge pull request #823 from rspec/let_before_all

Revise before(:all)/let behavior based on feedback from @dchelimsky.
Ensure let block is not passed to super call.

Fix #820. This is directly related to bug
http://bugs.ruby-lang.org/issues/8059.  It seems to affect Ruby 1.9.3
but not Ruby 2.0.0. As per the notes, it is a heisenbug, so simply
looking at it causes the bug to disappear.

Force the call to super to make sure the let block is not passed in.
Update Changelog with issue description.
Relax the ZenTest dependency.

4.6.2 will no longer install on travis because it requires rubygems ~> 1.8, and travis now has rubygems 2.0 installed.
Merge pull request #824 from pjg/readme_tweaks

Fix README example spec to display actual output
Fix travis CI build.

- Bundler 1.3 needs rbconfig to be required to run `bin/rspec` after
  installing with `--standalone --binstubs`.
  https://github.com/carlhuda/bundler/issues/2382
- The Travis VMs no longer have 32-bit JVMs available. Aruba 0.4.11
  forced the -d32 flag. 0.5 just documents it and makes a file available
  to require that will set it. We've updated Aruba here so that the jruby
  ENV vars aren't set automatically; we've kept the flag to disable JIT
  but removed -d32.
- Aruba now has a "the file xyz shoudl contain" step.
bin/cucumber needs rbconfig required, too.
rbconfig must be loaded before bundler/setup
I'm still getting undefined const RbConfig on travis.

I think this should fix it.
No need for this before_install script.
Allow a longer timeout on JRuby.
Merge pull request #827 from cupakromer/fix-issue-820

Ensure let block is not passed to super call.
Rephrase changelog entry so it focused on the what, not the how.

[ci skip]
Clarify changelog entry.

[ci skip]
Add missing changelog entry for #808.

[ci skip]
2.13.1 release.