Deprecate r-travis

[hadley]

I think we should start pro-actively deprecating the old r-travis system. Two reasonable first steps would be to:

1. Automatically file an issue on all repos that currently use it: https://github.com/search?utf8=✓&q=filename%3A.travis.yml+travis-tool.sh
2. Print a clear deprecation warning in the travis-tool script

[jimhester]

One issue with the code search is GitHub only includes the first 1000 repositories in its results.

I tried running some sample queries with google's bigquery dataset but the results were much lower than expected. I am guessing many R repositories are not included in the dataset because they do not have an explicit LICENSE file with an open source license, only a mention of it in the DESCRIPTION, so are not considered to be open source.

@craigcitro any idea if this is the case?