Improve Contributing to Rails Guide

Logically group the content, so it makes more sense if someone tries to read from start of page to end of page. [ci skip]
Use SVG version of travis build status badge [skip ci]
Merge pull request #14623 from t3chnoboy/master

Use SVG version of travis build status badge [ci skip]
Fix setup of adding _flash_types test.

Adding flash types to a controller within any of the tests will result
in a global state change of the controller under test.

This patch will prevent state leaks and allow us to run the test in random order.
Merge pull request #14614 from tgxworld/fix_add_flash_types_test

Fix setup of adding _flash_types test.
Added OS X specific commands to installation guide [ci skip]
Recommended using homebrew for installing MySQL and PostgreSQL
fix CollectionProxy delete_all documentation

This method no longer returns an array of all records that
have been removed. Correct documentation to reflect this change.

See issue 14546
Add documentation for Thread#freeze
Merge pull request #14626 from eileencodes/fix_incorrect_collection_proxy_delete_all_documentation

fix CollectionProxy delete_all documentation
Remove unused `subclass_controller_with_flash_type_bar` var from flash test.
Merge pull request #14624 from jamox/dev_install_guide

Added OS X specific commands to installation guide [ci skip]
Merge pull request #14628 from vipulnsward/remove-flash-test-warning

Remove unused `subclass_controller_with_flash_type_bar` var from flash test.
make enums distinct per class
Merge pull request #14609 from evanwhalen/enums_inheritance

Make enums distinct per class
Ignore order when doing count.

This is necessary because Postgresql doesn't play nice with ORDER BY and
no GROUP BY.

Fixes #14621.
Merge pull request #14631 from laurocaetano/fix_count_with_order

Ignore order when doing count.
Use connection-specific bytea escaping

In our normal usage, it's rare for this to make a difference... but is
more technically correct.

As well as a spec that proves this is a good idea, let's also add a more
sane-looking one that just covers basic to_sql functionality. There
aren't many places where we actually use escape_bytea, but that's one
that won't be going away.
Merge pull request #14632 from matthewd/escape_bytea

Use connection-specific bytea escaping
Build the reverse_order on its proper method.

The reverse_order method was using a flag to control if the order should
be reversed or not. Instead of using this variable just build the reverse order
inside its proper method.

This implementation was leading to an unexpected behavior when using
reverse_order and then applying reorder(nil).

Example:
  Before
    Post.order(:name).reverse_order.reorder(nil)
    # => SELECT "posts".* FROM "posts"   ORDER BY "posts"."id" DESC

  After
    Post.order(:name).reverse_order.reorder(nil)
   # => SELECT "posts".* FROM "posts"
Remove useless begin..end