Oembed warning if GD function is missing

Silencing errors from a function is terrible so I've stopped that.

I've also added detection for if `imagecreatefromstring` is a declared function and a useful error is thrown if not
DOCS: Minor fixes to RestfulService documentation
Fixed config setting link
Merge pull request #3076 from martinduparc/patch-4

Fixed config setting link
BUG Fix ImageTest
Image test would erroneously reset the Image::$backend to null if the test was skipped, breaking subsequent test cases
Merge pull request #3068 from kinglozzer/patch-1

DOCS: Minor fixes to RestfulService documentation
Removed demo folder
BUG Fix jstree when duplicating subtrees
Merge pull request #3031 from tractorcow/pulls/3.1-fix-tree-duplication

BUG Fix jstree when duplicating subtrees
Merge pull request #3078 from tractorcow/pulls/3.1-remove-thirdparty-demo

Removed demo folder
Merge pull request #3077 from tractorcow/pulls/fix-imagetest

BUG Fix ImageTest
FIX: corrected link to CMS Alternating Button Page
Merge pull request #3082 from PapaBearNZ/patch-1

FIX: corrected link to CMS Alternating Button Page
FIX: parent::init is not called first

In the controller's init function, parent::init() should be called before any other method calls.
Merge pull request #3083 from mparkhill/patch-1

FIX: parent::init is not called first
Merge pull request #3070 from dhensby/patch-3

Oembed warning if GD function is missing
FIX folder Title not being exactly the same as Name field if setName modified the value
Change PHP 5.6 to an allowed failure
Merge pull request #3086 from hafriedlander/fix/CWPBUG-155

FIX Folder Title not being exactly the same as Name field
BUG Allow PHPUnit installation with composer / Fix travis