Travis CI improvements.

[prioux]

I know this feature is not yet merged, but here are some thoughts about improvements to it.

• [x] rename the environment variable that Travis use to identify the docker image; instead of IMAGE_NAME maybe use something like CBRAIN_TRAVIS_IMAGE_NAME
• [x] seems the bash SECONDS counter doesn't increment as expected in travis.sh
• [x] maybe perform the dockerize steps for the template in the docker image instead of at run time?
• [x] document the CBRAIN_FAILTEST feature
• [x] use the --depth option with the git clone command inside the container? Value, 20 commits?
• [x] investigate if it would be possible to build the mysqldb inside the container and keep it in seeded mode right there
• [ ] document the distinction between the subdirectories cbrain_base, cbrain_travis and cbrain_test in the cbrain user HOME directory
• [x] customizing the Travis build from within .travis.yml (see https://docs.travis-ci.com/user/customizing-the-build/)
• [ ] make the docker image smaller by uninstalling things that are no longer needed at the end (compilers? libs? dev files?)

[prioux]

More improvements:

• [x] (DONE in #551) run the tests on the Bourreau side
• [ ] have a different docker image for the master branch and for the dev branch ?
• [ ] run tests for plugins! for this we'll need some new conventions:
  • have aspec directory in the root of a plugins package
  • have the .travis.yml there too
  • need new entry point in the base CBRAIN for running those tests
  • the cbrain_travis docker mountpoint will no longer exists, we need maybe to point one level up and make the script find out what plugin is being tested