[ticket/11324] Add PHP 5.5 environment to travis and allow it to fail.

PHPBB3-11324
Merge PR #1191 branch 'bantu/ticket/11324' into develop-olympus

# By Andreas Fischer
# Via Andreas Fischer
* bantu/ticket/11324:
  [ticket/11324] Add PHP 5.5 environment to travis and allow it to fail.
Merge PR #1191 branch 'develop-olympus' into develop

# By Andreas Fischer
# Via Andreas Fischer (1) and Oleg Pudeyev (1)
* develop-olympus:
  [ticket/11324] Add PHP 5.5 environment to travis and allow it to fail.