Fix for Rubinius

Problem
-------

Currently, CI is failing b/c singleton_class? was added to MRI2.1 but not Rbx.
e.g. https://travis-ci.org/pry/pry/builds/32133642

However, I did not look into why it is suddenly failing now.
For quite a while, Rbx builds were erroring out (e.g.  https://travis-ci.org/pry/pry/jobs/30421343)
They are no longer failing, but are now erroring, for the above reason.

Context
-------

Rbx has recently (11 days ago, 30 July 2014) received this method
https://github.com/rubinius/rubinius/commit/2e71722dba53d1a92c54d5e3968d64d1042486fe

But the functionality has been available since April 2011
https://github.com/rubinius/rubinius/commit/4310f6b2ef3c8fc88135affe697db4e29e4621c4

Solution
--------

Fix the failures by falling back to the older functionality if we are on Rubinius,
and the new method is unavailable.
Switch test suite to RSpec

Removes Bacon and Mocha

Mostly this went smoothly. There were a few errors that I fixed along
the way, e.g. tests that were failing but for various reasons still
passed. Should have documented them, but didn't think about it until
very near the end. But generaly, I remember 2 reasons this would happen:
`lambda { raise "omg" }.should.raise(RuntimeError, /not-omg/)` will pass
because the second argument is ignored by Bacon. And `1.should == 2`
will return false instead of raising an error when it is not in an it
block (e.g. if stuck in a describe block, that would just return false)

The only one that I felt unsure about was spec/helpers/table_spec.rb
`Pry::Helpers.tablify_or_one_line('head', %w(ing)).should == 'head: ing'`
This is wrong, but was not failing because it was in a describe block
instead of an it block.  In reality, it returns `"head: ing\n"`,
I updated the test to reflect this, though I don't know for sure
this is the right thing to do
Switch test suite to RSpec

Removes Bacon and Mocha

Reasoning explained in this comment: https://github.com/pry/pry/issues/277#issuecomment-51708712

Mostly this went smoothly. There were a few errors that I fixed along
the way, e.g. tests that were failing but for various reasons still
passed. Should have documented them, but didn't think about it until
very near the end. But generaly, I remember 2 reasons this would happen:
`lambda { raise "omg" }.should.raise(RuntimeError, /not-omg/)` will pass
because the second argument is ignored by Bacon. And `1.should == 2`
will return false instead of raising an error when it is not in an it
block (e.g. if stuck in a describe block, that would just return false)

The only one that I felt unsure about was spec/helpers/table_spec.rb
`Pry::Helpers.tablify_or_one_line('head', %w(ing)).should == 'head: ing'`
This is wrong, but was not failing because it was in a describe block
instead of an it block.  In reality, it returns `"head: ing\n"`,
I updated the test to reflect this, though I don't know for sure
this is the right thing to do

This will fail on master until https://github.com/pry/pry/pull/1281 is merged.
This makes https://github.com/pry/pry/pull/1278 unnecessary.
Run test in an object of its own

Otherwise we are sharing state with the toplevel binding,
which allows other tests to modify its values causing this to fail
e.g. https://travis-ci.org/pry/pry/jobs/32184681
Fix specs for JRuby
Merge branch 'fix-for-rubinius' into switch-to-rspec

I was asked to fix it for Rbx and JRuby
(https://github.com/pry/pry/pull/1282#issuecomment-51820884)
so decided to go ahead and merge this.

Conflicts:
	spec/commands/ls_spec.rb
	spec/commands/shell_command_spec.rb
Merge pull request #1282 from JoshCheek/switch-to-rspec

Switch test suite to RSpec