Configure travisci to release on Tag

[Manuel Martins]

Travis should be configured in order to deploy the jupyter extension to pypi and create the github release.

[Manuel Martins]

Implemented.
Use git flow for releases now.

v0.5.0 has been released this way:

mcmartins@local:~$ git checkout develop
mcmartins@local:~$ git flow init # all defaults except the tag prefix that should be 'v'
mcmartins@local:~$ git flow release start 0.5.0
mcmartins@local:~$ git checkout master
mcmartins@local:~$ git merge release/0.5.0
mcmartins@local:~$ git flow release finish '0.5.0'
mcmartins@local:~$ git push
mcmartins@local:~$ git push --tags