Match RStudio v1.2 embedded pandoc version of 2.2
Match RStudio v1.2 embedded pandoc version of 2.2

Increments the pandoc version to 2.2 from 1.15.2, which is embedded in RStudio v1.2.

This change was brought about because pandoc 1.15.2 is too low to be used bookdown.

Note: RStudio v1.1 has an embedded version of pandoc of 1.17.2
craigcitro/travis-build,andyli/travis-build,craigcitro/travis-build,andyli/travis-build