Modernize the PHP-CS-Fixer rules

[NicolasCARPi]

• Require PHP-CS-Fixer as dev dependency
• Modify .gitignore to ignore php_cs.cache and generated coverage files
• Run the "fix" command on the codebase

[NicolasCARPi]

Note that the tests are failing with travis for an unrelated reason.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.

Project coverage is 89.72%. Comparing base (13fe6f8) to head (933e9dc). Report is 100 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Core.php	75.00%	1 Missing :warning:
src/KeyProtectedByPassword.php	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #441 +/- ## ======================================= Coverage 89.72% 89.72% ======================================= Files 9 9 Lines 925 925 ======================================= Hits 830 830 Misses 95 95 ```

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