Upgrade Guzzle and other dependencies

[NamelessCoder]

This patch:

• Upgrades the Guzzle dependency to ^6
• Upgrades secondary dependencies to most recent major version minimum
• Removes php-coveralls which is unused
• Removes php-documentor that should be global on host generating docs
• Replaces some string class names with ::class for easier tracking while upgrading
• Removes extension-less content stream rewriting
• Fixes method calls with undeclared arguments

[NamelessCoder]

Breaking change: API of Client class is different when using custom HTTP invoker as illustrated in examples

I apologise for the combined commit which Ideally should have been several smaller commits. The only excuse I have is the dependency puzzle was a lot harder to solve without removing those dependencies - and detecting usages of classes was very difficult due to the classname-in-string occurrences.

Caveats:

• I had to remove the "content stream without extension" mime type fix - I'm not sure how to reproduce that or even if it needs reproducing. I had no proper test case to confirm this, hope you may be able to help there.
• The behavior of ->json on Response is now mimicked with several json_decode calls and casts to array. This only mimics the behavior and doesn't throw JSON-specific exceptions. Instead, data integrity exceptions will be thrown. No test cases cover this.

Suggested next steps:

• Switch league/url to league/uri - I had initially attempted to include this switch as well, but opted to avoid it to keep the change smaller (it's big enough already...)
• Drop support for PHP5 branches and implement null coalesce operators (many potential cases)
• Upgrade PHPUnit dependency (version depends on decision to drop PHP5 branch support)