Simplified the Travis configuration

Submodules are already handled by Travis itself, so no need to do it a second time
minor #3711 Simplified the Travis configuration (stof)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #3711).

Discussion
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Simplified the Travis configuration

Submodules are already handled by Travis itself, so no need to do it a second time

This should be applied in all branches where the Travis config is available

Commits
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b3a5660 Simplified the Travis configuration