Add Travis CI to do unit tests on functions

[erictleung]

TODO:

• Check that package builds correctly
• Install dada2HPCPipe package correctly and load successfully
• Run through code workflow
• Automate code linting with lintr https://github.com/jimhester/lintr

Resources:

• https://docs.travis-ci.com/user/languages/r
• https://blog.rstudio.com/2016/03/09/r-on-travis-ci/

Draft:

language: r
cache: packages
bioc_packages:
  - dada2
  - phyloseq
r_packages:
  - dplyr