[TASK] Travis: increase PHP memory_limit to 1280m for unit tests

Currently travis is failing due to exploded memory_limit (>1024m).
While we don't fix the memory leak, we rise the limit a bit to make
it run through again.

Resolves: #54511
Releases: 6.2
Change-Id: I10b7a8c3bdd64db3adddba84e17edb85464e51db