Add continuous integration

[dmcglinn]

Hey @FelixMay I just added continuous integration on the mobr repo it was pretty easy. If you'd like I can set this up for this repo as well. See the .travis.yml file: https://github.com/MoBiodiv/mobr/blob/master/.travis.yml
and linked tag on README.md: https://github.com/MoBiodiv/mobr/blob/master/README.md

[FelixMay]

Hi @dmcglinn, could you give me some hints or a link with explanations what the continuous integration does and why it is useful?

[dmcglinn]

so basically it runs devtools::check and reports if there are any errors every time you merge changes to master. This is useful because it is a bit of a pain to do this on your local machine and you can get an email telling you if the merge caused the check to fail and why.