PgSearch::Document searchable_type fix for STI

searchable_type contained root class for STI subclass
This commit fixes this bug, searchable_type now contains subclass type

See @elbouillon comments https://github.com/Casecommons/pg_search/issues/59
Add Rails 4.1 to Travis build

Also add 4-1-stable branch and allow failures.
Don't run tests against ruby 2.1.0 anymore

It always segfaults.
Added syntax highlighting
Merge pull request #165 from markijbema/patch-1

Add syntax highlighting
Add support for non-standard primary keys
Add support for non-standard primary keys
Add spec for non-standard primary key for #169
Merge pull request #169 from mattbeedle/primary-key

Add support for non-standard primary keys
Catch ActiveRecord::NoDatabaseError in spec_helper
VERSION 0.7.4
Convert specs to RSpec 2.14.8 syntax with Transpec

This conversion is done by Transpec 2.0.0 with the following command:
    transpec

* 130 conversions
    from: obj.should
      to: expect(obj).to

* 70 conversions
    from: == expected
      to: eq(expected)

* 33 conversions
    from: obj.should_not
      to: expect(obj).not_to

* 16 conversions
    from: obj.should_receive(:message)
      to: expect(obj).to receive(:message)

* 14 conversions
    from: =~ [1, 2]
      to: match_array([1, 2])

* 9 conversions
    from: obj.should_not_receive(:message)
      to: expect(obj).not_to receive(:message)

* 9 conversions
    from: obj.stub(:message)
      to: allow(obj).to receive(:message)

* 4 conversions
    from: > expected
      to: be > expected

* 2 conversions
    from: its(:attr) { }
      to: describe '#attr' do subject { super().attr }; it { } end

* 1 conversion
    from: =~ /pattern/
      to: match(/pattern/)

* 1 conversion
    from: obj.stub(:message).and_return { value }
      to: obj.stub(:message) { value }

See also: https://github.com/yujinakayama/transpec#supported-conversions
Stop supporting should syntax in RSpec
Idiomatic Markdown linking
Use SVG badges from shields.io where not natively provided
Break out spec_helper into separate files
Update stubbing in specs to new RSpec "expect" style
Do a nil check, as the association may no longer be present

(#177)
Lock RSpec to 2.x
Migrate completely to RSpec allow/expect syntax