Fix failing specs caused by $SAFE incompatibility on MRI 2.3

Remove nokogiri development dependency
Conflicts:
	rspec-core.gemspec

MRI 2.3 has only $SAFE level 1, so File.expand_path('.') does not raise
SecurityError since the '.' is not tainted.

... because MRI 2.3 does not have $SAFE level to mark every object as
tainted.

... because the matcher may be autoloaded if it's used first time in the
process and raises SecurityError.

…01-06-for-3-4-maintenance

Updates from rspec-dev (2016-01-06)

In the case where RSpec does not handle an error (e.g. SystemExit), we
still want to capture the example timing data to not break profiling
information that may get generated via `--profile`.

Fixes #2142

If we start the spec run in a `rescue` block, ruby will set the
rescued exception to be the cause of all exceptions that occur
during the run.

…ot on your path.

Apparently, `::Kernel.require` is _only_ ruby's built-in require
(based on the `$LOAD_PATH`) and does not get the enhanced capabilities
of rubygems when rubygems is loaded. Rubygems only monkey patches
`::Kernel#require`, not `::Kernel.require`.

Originally, I had used `::Kernel.require` to load coderay to make
it easier to stub `require` to simulate coderay not being available,
as it allowed me to treat coderay as a collaborator. Using bare
`require` instead forces me to stub the object-under-test, which is
a bit smelly, but it fixes things so that you can get syntax
highlighting without using something like bundler to manage your
project and setup load paths.

Compare:

$ ruby -rubygems -e "p method(:require).source_location"
["/Users/myron/.rubies/ruby-2.1.5/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb", 38]
$ ruby -rubygems -e "p ::Kernel.method(:require).source_location"
nil