https://docs.travis-ci.com/user/multi-os/#Example-Multi-OS-Build-Matrix

before installing the gsl R package.  Related to https://github.com/cran/gsl/blob/master/INSTALL and to https://stackoverflow.com/questions/49162670/r-package-build-failing-on-unix-machines-due-to-missing-gsl-gnu-scientific-lib/49165163#49165163

"These installs will be faster than source installs, but may not always be the most recent version"
https://docs.travis-ci.com/user/languages/r/#Additional-Dependency-Fields

with `r-cran-qqq`.  Also needs sudo.

> "Specify the name just as you would when installing from CRAN. On OS X builds and builds without sudo: required, these packages are installed from source."

https://docs.travis-ci.com/user/languages/r/#Additional-Dependency-Fields

I bet the order isn't exactly right though.

it apparently isn't available

so it won't install from other R packages from source, before their binaries can be installed.  Remove OpenMX b/c it was failing on the OSX builds

to keep the dependency calculations simpler.  Something's getting tripped up with OpenMX

I don't know why Travis is looking for [these packages](https://travis-ci.org/IndrajeetPatil/ggstatsplot/jobs/353142095#L1620).  It's not exactly the same list.

```
#  - rpf
#  - openssl
#  - rlang
#  - igraph
#  - utf8
#  - gss
#  - haven
#  - XML
#  - data.table
#  - matrixStats
#  - rgl
#  - dplyr
#  - purrr
#  - tidyver
#  - readr
#  - minqa
#  - mvtnorm
#  - SparseM
#  - lme4
#  - httpuv
#  - markdown
#  - sem  
#  - readxl
#  - openxlsx
#  - pander
#  - minpack.lm
#  - StanHeaders
```