username for dockerhub from env. variables

[MartinHinz]

With this the username for hub.docker.com is taken from the travis env variables instead of taken the github user name.

Referring to our discussion in https://github.com/benmarwick/rrtools/issues/12, this actually seems to be more elegant than entering it locally since:

• When using travis, this environmental variable must be given anyhow
• to work at all, it must be correct
• we cannot script this from within R
• less redundancy

[benmarwick]

Thanks, yes, very elegant! Excellent idea! The could be a problem in the case where a user wants to use docker without travis (for example make a docker container locally on their computer), but let's not worry about that for now because that has other issues to consider also.

[MartinHinz]

The only file affected is the travis.yml, while the dockerfile itself does not contain any information about the user. So there should be no problems in respect to this.? What about closing #12 then?

[benmarwick]

Thanks, closes #12