fix one of the possible causes of build failures

- _merge_request partial truncates the title of the merge request to 80 char ('first 80 char' + '...')
- ```page should_have mr.title``` searches the page against the full title

Sometimes, merge request is generated with a title with more than 80 char and spec fail because it can't find the trucated title as it searches the untrucated title. 

See [origial travis failure](https://travis-ci.org/gitlabhq/gitlabhq/builds/3593153).