Add support for scc merging in Travis

- Add a new `scc travis-merge` command which relies on environment variables and PR comments to set all the parameters
- Call this command before running the script in Travis.yml

Testing scenarios:
- find a openmicroscopy PR number pr opened from `my_user/my_branch`

 ```
git fetch my_user
git checkout my_user/my_branch
TRAVIS_PULL_REQUEST=pr python ~/code/scc/scc.py travis-merge
```
 should bump the submodules

- add a comment ``--depends-on pr2`` to the PR

 ```
git checkout my_user/my_branch
TRAVIS_PULL_REQUEST=pr_number python ~/code/scc/scc.py travis-merge
```
 should bump the submodules and merge pr2 locally

- add a comment ``--depends-on pr3`` to the PR

 ```
git checkout my_user/my_branch
TRAVIS_PULL_REQUEST=pr_number python ~/code/scc/scc.py travis-merge
```
 should bump the submodules and merge pr2 and pr3 locally