Add 2.1.1 to travis
Remove unused variable from Rakefile
Use newer hash style
Correctly pass command line options to minitest
Minor reorder of variable declaration
Fix post_mortem mode

And add very simple tests for now. Still need to check commands one by
one.
Fix typo
Fix some whitespace
Rename variable
Allow disabling post_mortem mode
Much simple debug_file method
Only declare variable when it's used
Set exception only when necessary
Remove unnecessary flag
More compact variable declaration
Rename some variables to be more readable
Remove redundant RuntimeError's
Fix (finally) post_mortem mode

The change is ruby core that was breaking this was
[r43561](https://bugs.ruby-lang.org/projects/ruby-trunk/repository/revisions/43561).
rb_errinfo is not set anymore in TracePoint API events, so we were not
correctly setting the exception and context information into it. The
correct way to set the exception in the raise event is actually throught
the TracePoint API, provided by the raised_exception method.

Also, $! seemed to be nil in the at_exit hook. I think this is because
$! is not set from rb_errinfo if there's not rescue or ensure block
(like it happens in post_mortem mode). To solve this I use a class
variable to keep the raised_exception instead of relying on $! (this
change was actually made in 1cbf5d2).
Fix last post_mortem test

We don't need to rescue and reraise in debug_file test method, an ensure
clause is enough. Not doing this has the benefit that the last raised
exception is the exception set in the original raise event and not the
one set in the reraise" event, so we can make assertions on it.
Check ruby version before anything