This brings support for the NETRC environment variable, which
makes it easier to isolate the unit tests from the users's
own netrc file.

This fixes the following warning that appears when you run
`python setup.py test`:

```
=============================== warnings summary ===============================
ligo/gracedb/rest.py:1203
  /Users/lpsinger/src/gracedb-client/ligo/gracedb/rest.py:1203: DeprecationWarning: invalid escape sequence \
    """  # noqa: W605

ligo/gracedb/rest.py:1300
  /Users/lpsinger/src/gracedb-client/ligo/gracedb/rest.py:1300: DeprecationWarning: invalid escape sequence \
    """  # noqa: W605

ligo/gracedb/rest.py:2227
  /Users/lpsinger/src/gracedb-client/ligo/gracedb/rest.py:2227: DeprecationWarning: invalid escape sequence \
    """  # noqa: W605

-- Docs: https://docs.pytest.org/en/latest/warnings.html
================ 488 passed, 98 deselected, 3 warnings in 5.63s ================
```

* Python 3.4 reached end of life on March 18, 2019.
* The ligo-gracedb unit tests do not pass under Python 3.4.

Adding integration tests in support of the RAVEN superevent
field and VOEvent changes.

adding combined_skymap_filename (optional string)  and raven_coinc (optional boolean) to createVOEvent method

Update .gitlab-ci.yml: the default python3 version was changed in the ligo/base:el7 container, so the build dependencies had to be changed to reflect that. 

According to the classifiers in setup.py, Python 2.6 is no longer
supported by this package.

Add the ability to print the certificate's subject and expiration
date/time.  Add information on using these new functions to the
troubleshooting docs.  Add unit tests.

README.md file doesn't seem to come along for the ride when
system packages are built, so we converted to .rst format,
since that seems to work.

Hasn't been used on the production server since 2015, actual
usage was only very minimal. The model and related endpoints will
be removed from the server in the near future.

fixes #22

Add update_grbevent method to ligo.gracedb.rest.GraceDb. Used for
updating parameters of GRB events.