Improve the discovery of submodules in the build script

[jonasraoni]

Describe the problem you would like to solve
At this moment we have to create a commit containing the text ##github-username/branch-name## to hint the build script where to look for updated submodules.

Describe the solution you'd like
Update the script startSubmodulesTRAVIS.sh to look for submodules/matching hashes automatically at the PR's origin.

Who is asking for this feature?
Developer

[henriqueramos]

Already made on #7125.

https://github.com/pkp/ojs/pull/3160/files#diff-8f16e27c2e43ab2ebfaa7bf22fb77c3be7e92671fc0d73cc33ef8c47517d0df5

[jonasraoni]

I created this issue to go one step forward, and avoid the need of creating a commit with the message ##github-username/branch-name## =]