Check merge request on push

When we receive a push we need to test whatever the branch this is part of a open merge request as in order to perform a new build of the merge request. 

Currently I have implemented this using the GitLab api to query for open merge request. And check if the newly pushed branch is the source of one of this requests. 
Should we also check if if is the target branch we pushes to?


Note: This pull request depends on one of my other pull request #34 as I need to create a new GitLabMergeRequest to trigger the build.