retry fix to wget for fragile MacTEX downloads
retry fix to wget for fragile MacTEX downloads

Add `wget --retry-on-http-error=403 --waitretry=3 --tries=5` to the command for downloading MacTeX from CTAN to address fragile downloads (makes R travis jobs fail something like a third of the time under some settings). See https://github.com/travis-ci/travis-ci/issues/6702 for discussion.
andyli/travis-build,craigcitro/travis-build,craigcitro/travis-build,andyli/travis-build