Enable testing of retriever interface functions on Travis CI

[ha0ye]

MATSS has functions that obtain and load data using retriever and the associated rdataretriever R package. We would like to test those functions.

This means, we'll need to get retriever properly installed on the Travis CI instance. (rdataretriever is handled easily by the R package dependency functionality)

Currently, we are able to get python3 and retriever installed properly. The trick is to point the reticulate R package (which rdataretriever uses to interface retriever with R) to use the correct python version. Here's current output from reticulate's config on Travis:

python               : /usr/bin/python
libpython            : /usr/lib/python2.7/config-x86_64-linux-gnu/libpython2.7.so
pythonhome           : /usr:/usr
virtualenv_activate  : 
version_string       : 2.7.6 (default, Nov 23 2017, 15:49:48)  [GCC 4.8.4]
version              : 2.7
architecture         : 64bit
anaconda             : FALSE
numpy                : NULL
required_module      : NULL
required_module_path : NULL
available            : TRUE
python_versions      : c("/usr/bin/python", "/usr/bin/python3")
forced               : NULL

[ha0ye]

closed by #121