Add mysql testdb

[henrykironde]

RMySQL has been replaced by RMariaDB
ropensci#149

[henrykironde]

@pranita-s the tests look fine now(merge this to see how it runs).
Clean up the travis file and add comments where applicable. You could try installing these packages

  - R -e 'install.packages(c("devtools"));if (!all(c("devtools") %in% installed.packages())) { q(status = 1, save = "no")}'
  - R -e 'install.packages(c("devtools"));if (!all(c("devtools") %in% installed.packages())) { q(status = 1, save = "no")}'
  - R -e 'library(devtools);devtools::install_github("klutometis/roxygen")'
  - R -e 'library(devtools);devtools::install_github("rstats-db/DBI")'
  - R -e 'library(devtools);devtools::install_github("rstats-db/RMariaDB")'
  - R -e 'devtools::install_github("rstudio/reticulate")'

using

r_github_packages:
  - r-lib/devtools
  - klutometis/roxygen
  - rstats-db/DBI
  - rstats-db/RMariaDB
  - rstudio/reticulate

Also try removing the script: section so that R can use the default test procedures.
This is because if we run

- R -e 'library(testthat)'
- R -e 'library(devtools);devtools::test()'

The tests do not show failures.
ref :r-lib/testthat#700