Upgrade to RSpec 3.0.

For this I've enabled both the old expectation and stubbing/mocking syntax. The
old syntax is much more compact and to me reads nicer. For example, consider
the following:

    lex('<foo></foo>').should == [...]

To me this reads much nicer than this:

    expect(lex('<foo></foo>')).to eq([...])

oga.gemspec

@@ -37,7 +37,7 @@ Gem::Specification.new do |s|
   s.add_dependency 'ast'
 
   s.add_development_dependency 'rake'
-  s.add_development_dependency 'rspec'
+  s.add_development_dependency 'rspec', ['~> 3.0']
   s.add_development_dependency 'yard'
   s.add_development_dependency 'simplecov'
   s.add_development_dependency 'kramdown'

spec/spec_helper.rb

@@ -11,4 +11,12 @@ require_relative 'support/parsing'
 RSpec.configure do |config|
   config.color = true
   config.include Oga::ParsingHelpers
+
+  config.expect_with :rspec do |c|
+    c.syntax = [:should, :expect]
+  end
+
+  config.mock_with :rspec do |c|
+    c.syntax = [:should, :expect]
+  end
 end