Avoid calls to Proc#bind (deprecated in Rails 4.x) when instance_exec available

Calls to Proc#bind in shoulda-context in a Rails 4.x project result in hundreds of instances of e.g.:
```
DEPRECATION WARNING: Proc#bind is deprecated and will be removed in future versions. (called from bind at /Users/ryan/.rbenv/versions/jruby-1.7.4/lib/ruby/gems/shared/gems/activesupport-4.0.1/lib/active_support/core_ext/proc.rb:6)
/Users/ryan/.rbenv/versions/jruby-1.7.4/lib/ruby/gems/shared/gems/activesupport-4.0.1/lib/active_support/core_ext/proc.rb:6:in `bind'
  /Users/ryan/.rbenv/versions/jruby-1.7.4/lib/ruby/gems/shared/gems/shoulda-context-1.1.5/lib/shoulda/context/context.rb:424:in `run_all_teardown_blocks'
```
In test runs with deprecation warnings/traces turned on.