Update .travis.yml

Add PHP 5.5 to Travis-CI testing and allow it to fail.
[Form] Fix failing `MonthChoiceList` in PHP 5.5
[Locale] Fix failing `StubIntlDateFormatter` in PHP 5.5
[Locale] Fix failing `StubIntlDateFormatter` tests in PHP 5.5
[Locale] Adjust `StubIntlDateFormatter` to have new methods added in PHP 5.5
merged branch stloyd/feature/travis_php55 (PR #6552)

This PR was merged into the 2.0 branch.

Commits
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73d9cef [Locale] Adjust `StubIntlDateFormatter` to have new methods added in PHP 5.5
b2ce983 [Locale] Fix failing `StubIntlDateFormatter` tests in PHP 5.5
913b564 [Locale] Fix failing `StubIntlDateFormatter` in PHP 5.5
8ae773b [Form] Fix failing `MonthChoiceList` in PHP 5.5
f4ce2f1 Update .travis.yml

Discussion
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[2.0] Add testing of PHP 5.5 on Travis-CI & fix failing code