change travis file to use newer R lang setup and cache: packages, sudo: false to speed up builds

.travis.yml

-language: c
-script: "./travis-tool.sh run_tests"
-before_script:
-- curl -OL http://raw.github.com/craigcitro/r-travis/master/scripts/travis-tool.sh
-- chmod 755 ./travis-tool.sh
-- "./travis-tool.sh bootstrap"
-- "./travis-tool.sh install_deps"
+language: r
+sudo: false
+cache: packages
 notifications:
   slack:
     secure: PcdJMRncDb0dO4bGZLc8tcjhakOdhglyNQr8acdL1Wrq23Qb+ZF6fFfaahrbpSRD+pdpnXUXkmhWjbuqfHY1mnvC9gsFMr9KD9P82hJs2luRUs+mZBKWU45v4puAm19Zepr0P/rsj0KUJmcJepsxLo1ccjOY0xSxzf7jvyf8lx0=