ARROW-3804: [R] Support older versions of R runtime
ARROW-3804: [R] Support older versions of R runtime

`vctrs` requires `3.1`: https://github.com/r-lib/vctrs/blob/master/DESCRIPTION#L15, but no functionality makes use of `3.5`, at least, not yet.

Fix for: https://issues.apache.org/jira/browse/ARROW-3804

Author: Javier Luraschi <javierluraschi@hotmail.com>
Author: Romain Francois <romain@rstudio.com>

Closes #2978 from javierluraschi/bugifx/r-version and squashes the following commits:

a0f153f10 <Romain Francois> testing on released R
105f6a187 <Javier Luraschi> also match travis r version
1fcf99a1e <Javier Luraschi> lower requirement of r runtime

apache/arrow,laurentgo/arrow,majetideepak/arrow,cpcloud/arrow,laurentgo/arrow,apache/arrow,cpcloud/arrow,laurentgo/arrow,renesugar/arrow,wesm/arrow,majetideepak/arrow,kou/arrow,icexelloss/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,xhochy/arrow,renesugar/arrow,cpcloud/arrow,laurentgo/arrow,renesugar/arrow,kou/arrow,cpcloud/arrow,apache/arrow,apache/arrow,renesugar/arrow,xhochy/arrow,majetideepak/arrow,apache/arrow,kou/arrow,laurentgo/arrow,majetideepak/arrow,renesugar/arrow,majetideepak/arrow,xhochy/arrow,icexelloss/arrow,renesugar/arrow,cpcloud/arrow,icexelloss/arrow,majetideepak/arrow,majetideepak/arrow,laurentgo/arrow,wesm/arrow,wesm/arrow,majetideepak/arrow,xhochy/arrow,renesugar/arrow,apache/arrow,kou/arrow,renesugar/arrow,cpcloud/arrow,wesm/arrow,kou/arrow,majetideepak/arrow,wesm/arrow,laurentgo/arrow,majetideepak/arrow,icexelloss/arrow,apache/arrow,cpcloud/arrow,icexelloss/arrow,xhochy/arrow,kou/arrow,kou/arrow,icexelloss/arrow,majetideepak/arrow,apache/arrow,xhochy/arrow,xhochy/arrow,wesm/arrow,cpcloud/arrow,kou/arrow,laurentgo/arrow,renesugar/arrow,wesm/arrow,wesm/arrow,wesm/arrow,xhochy/arrow,apache/arrow,wesm/arrow,kou/arrow,wesm/arrow,xhochy/arrow,laurentgo/arrow,laurentgo/arrow,cpcloud/arrow,icexelloss/arrow,wesm/arrow,kou/arrow,renesugar/arrow,apache/arrow,kou/arrow,majetideepak/arrow,cpcloud/arrow,renesugar/arrow,xhochy/arrow,apache/arrow,xhochy/arrow,apache/arrow,majetideepak/arrow,apache/arrow,renesugar/arrow,icexelloss/arrow,xhochy/arrow,renesugar/arrow,kou/arrow,cpcloud/arrow,cpcloud/arrow,kou/arrow,icexelloss/arrow,wesm/arrow,majetideepak/arrow,laurentgo/arrow,majetideepak/arrow,cpcloud/arrow,icexelloss/arrow,laurentgo/arrow,icexelloss/arrow,renesugar/arrow,xhochy/arrow,laurentgo/arrow,wesm/arrow,renesugar/arrow,laurentgo/arrow,xhochy/arrow,icexelloss/arrow,kou/arrow,cpcloud/arrow,xhochy/arrow,apache/arrow