Update phpunit/phpunit requirement from 5.5.* to 5.7.*

[dependabot]

Updates the requirements on phpunit/phpunit to permit the latest version.

Changelog

Sourced from phpunit/phpunit's changelog.

[5.7.27] - 2018-02-01

Fixed

• Fixed #2236: Exceptions in tearDown() do not affect getStatus()
• Fixed #2950: Class extending PHPUnit\Framework\TestSuite does not extend PHPUnit\FrameworkTestCase
• Fixed #2972: PHPUnit crashes when test suite contains both .phpt files and unconventionally named tests

[5.7.26] - 2017-12-17

Fixed

• Fixed #2472: PHPUnit\Util\Getopt uses deprecated each() function
• Fixed #2833: Test class loaded during data provider execution is not discovered
• Fixed #2922: Test class is not discovered when there is a test class with @group and provider throwing exception in it, tests are run with --exclude-group for that group, there is another class called later (after the class from above), and the name of that another class does not match its filename

[5.7.25] - 2017-11-14

Fixed

• Fixed #2859: Regression caused by fix for #2833

[5.7.24] - 2017-11-14

Fixed

• Fixed #2833: Test class loaded during data provider execution is not discovered

[5.7.23] - 2017-10-15

Fixed

• Fixed #2731: Empty exception message cannot be expected

[5.7.22] - 2017-09-24

Fixed

• Fixed #2769: Usage of setUseErrorHandler() produces Undefined variable error

[5.7.21] - 2017-06-21

Added

• Added PHPUnit\Framework\AssertionFailedError, PHPUnit\Framework\Test, and PHPUnit\Framework\TestSuite to the forward compatibility layer for PHPUnit 6

Fixed

• Fixed #2705: stderr parameter in phpunit.xml always considered true

... (truncated)

Commits

• b7803ae Prepare release
• 63f3e4a Update Composer to 1.6.3
• 023f4f0 Allow failures on PHP 7.2 and PHP 7.3
• e92a128 Update ChangeLog
• d125b4e Make this compatible with PHPUnit 5.7
• 63f2fec code style requires a namespace
• 20ffc44 typo
• 853e43c Fix: PHPUnit shouldn't fail when a test suite contains both *.phpt files and ...
• 7d1494a Update tools
• 32f4c99 Update Composer to 1.6.2
• Additional commits viewable in compare view

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[dependabot]

OK, I won't notify you again about this release, but will get in touch when a new version is available. If you'd rather skip all updates until the next major or minor version, let me know by commenting @dependabot ignore this major version or @dependabot ignore this minor version. You can also ignore all major, minor, or patch releases for a dependency by adding an ignore condition with the desired update_types to your config file.

If you change your mind, just re-open this PR and I'll resolve any conflicts on it.

[pfefferle]

@dependabot ignore this major version

[dependabot]

OK, I won't notify you about version 5.x.x again, unless you re-open this PR or update to a 5.x.x release yourself.