Add scc travis-merge to the Travis before_install step (rebased onto dev_4_4)



This is the same as gh-1222 but rebased onto dev_4_4.

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This PR integrates the scc features added in openmicroscopy/snoopycrimecop#77, openmicroscopy/snoopycrimecop#79 and openmicroscopy/snoopycrimecop#81 into the openmicroscopy Travis build.

Before Travis builds the server and runs `test-compile`, `scc travis-merge` should be called which performs the following actions:
- bumps the submodules to the HEAD of `origin/$branch`
- merges all the PRs specified by ``--depends-on #prnumber`` or ``--depends-on repo/name#prnumber`` in the comments of the PR