add Pry::LastException.

pry adds behavior(methods) to the exception it returns at _pry_.last_exception
in a repl but when the exception has been passed along as a frozen object ruby
will raise an exception and pry fails to function as usual.

the commit changes that behavior so that the exception is wrapped in an instance of
Pry::LastException who is a subclass of BasicObject. Pry::LastException is a copy&paste
of what was defined dynamically in pry_instance.rb but it allows for frozen exceptions
to remain frozen and still provides the methods pry dynamically defined on an exception
object before. through a wrapper class pry also no longer modifies an exception object
that it doesn't own.
update CHANGELOG.md
Merge pull request #1145 from robgleeson/fix-last_exception

Add Pry::LastException class wrapping last_exception
Unwrap last exception when setting _ex_
Remove version guard from LastException check

In the happy path, the object is a RuntimeError and therefore inherits
from Object. In the sad path, failing when we try to bind the method is
just as informative as failing the actual assertion.
update CHANGELOG.md

'openstruct' is not a runtime dependency that pry has anymore but it
was not added to the changelog. other two changes include better messages
for older changelog entries related to config.