Add static analysis with phpstan

[codecov[bot]]

Codecov Report

Merging #369 (fb764b8) into master (465be0c) will decrease coverage by 2.11%. The diff coverage is 75.75%.

@@             Coverage Diff              @@
##             master     #369      +/-   ##
============================================
- Coverage     90.62%   88.51%   -2.11%     
- Complexity      150      156       +6     
============================================
  Files             1        1              
  Lines           448      418      -30     
============================================
- Hits            406      370      -36     
- Misses           42       48       +6     

Impacted Files	Coverage Δ
src/Selenium2Driver.php	88.51% <75.75%> (-2.11%)	:arrow_down:

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