Remove oldrel from Travis CI build matrix

[tomschenkjr]

The core development team has formalized a policy that RSocrata need only to be compatible with the most current release of R.

Currently, all commits of RSocrata are being tested on three versions of R on Travis CI: (1) the most recent release (release), (2) the beta version of R (devrel), and (3) the penultimate version of R (oldrel).

Item (3) may be problematic since it may fail due to package incompatibility or using base-R functions not available in the penultimate release, which we no longer require support for. Thus, to maintain consistency with our policy that RSocrata must pass all unit tests, we should remove tests for oldrel from the Travis CI build matrix.