Update dependencies and tooling

[phil-davis]

• drop PHP 7.0
• move to phpunit 7
• use later phpstan
• run php-cs-fixer in CI

[phil-davis]

The newer version of phpstan reports:

------ ------------------------------------------------------- 

  Line   Reader.php                                             

 ------ ------------------------------------------------------- 

  187    Unreachable statement - code above always terminates.  

 ------ ------------------------------------------------------- 

 ------ ---------------------------------------------------------------------- 

  Line   Serializer/functions.php                                              

 ------ ---------------------------------------------------------------------- 

  167    Call to function is_null() with array|object will always evaluate to  

         false.                                                                

         💡 Because the type is coming from a PHPDoc, you can turn off this     

         check by setting treatPhpDocTypesAsCertain: false in your             

         /home/travis/build/sabre-io/xml/phpstan.neon.                         

 ------ ---------------------------------------------------------------------- 

 ------ ----------------------------------------------------------------- 

  Line   Writer.php                                                       

 ------ ----------------------------------------------------------------- 

  130    Parameter #3 $uri of method XMLWriter::startElementNs() expects  

         string, null given.                                              

 ------ ----------------------------------------------------------------- 

 [ERROR] Found 3 errors                                                         

I am looking at those.

[codecov[bot]]

Codecov Report

Merging #173 into master will not change coverage. The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #173   +/-   ##
=========================================
  Coverage     97.61%   97.61%           
+ Complexity      114      112    -2     
=========================================
  Files            13       13           
  Lines           461      461           
=========================================
  Hits            450      450           
  Misses           11       11

Impacted Files	Coverage Δ	Complexity Δ
lib/Writer.php	100% <ø> (ø)	19 <0> (ø)	:arrow_down:
lib/Service.php	100% <ø> (ø)	18 <0> (-2)	:arrow_down:
lib/LibXMLException.php	100% <ø> (ø)	2 <0> (ø)	:arrow_down:
lib/Serializer/functions.php	100% <100%> (ø)	0 <0> (ø)	:arrow_down:
lib/Reader.php	99.13% <100%> (+0.01%)	43 <0> (ø)	:arrow_down:

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