Stop trying to run PHP 5.6 tests on Travis CI

Mockery has issues on PHP 5.6. The package should still be compatible,
but it can't be tested until Mockery is updated.

Revert "Try running tests on PHP 5.6 on Travis CI"

This reverts commit a52209931298cf0656545947b5cbda8b6f4697ab.
Add CaseAttachment Create/List/Show operations
Add missing case_id to CreateCaseAttachment
Extend content from parameter

This absence prevented the content field from showing up in the curl request
Fix test