Use firefox headless instead of phantomjs

[ocefpaf]

Closes #891

I chose firefox b/c it is easier to test on Travis-CI and we found some rendering issues with chrome/chromium in the past.

PS: @Conengmo the _png() is not a first class citizen yet. Not sure what else we could do to make it more robust. Maybe a driver selection option?

[ocefpaf]

This is a bad rebase :-/ Let me fix those.

[ocefpaf]

@Conengmo rebased and ready for a review. This one is not too important b/c we don't really advertise this functionality yet.

[Conengmo]

Looks good, and the tests work fine on Travis. Only problem is that the temporary file doesn't work on Windows. We had this problem before:
https://docs.python.org/3.6/library/tempfile.html#tempfile.NamedTemporaryFile

I'll see if I can find a solution, otherwise we'll merge it as is.

[Conengmo]

I can get the tempfile stuff to work if instead of using NamedTemporaryFile we use the underlying mkstemp function. We have to close and delete the file ourselves, but it's not that hard.

@contextmanager
def _tmp_html(data):
    """Yields the path of a temporary HTML file containing data."""
    filepath = ''
    try:
        f_int, filepath = tempfile.mkstemp(suffix='.html', prefix='folium_')
        os.write(f_int, data.encode('utf8'))
        os.close(f_int)
        yield filepath
    finally:
        if os.path.isfile(filepath):
            os.remove(filepath)

Note that this returns the file path, not a file descriptor. What do you think @ocefpaf?

[ocefpaf]

Note that this returns the file path, not a file descriptor. What do you think @ocefpaf?

I'm fine with that solution. (Actually I used something similar in the past when writing netCDF files, my bad for ignoring Windows here.)

[ocefpaf]

@Conengmo I'll take a look at the failures later but it seems that only Python 2.7 is affected and, b/c this feature is not really ready for prime time, I'm OK making it Python 3 only. (We already agreed that the next release is the last one for Python 2.7 anyway.)

[stickler-ci]

E302 expected 2 blank lines, found 1

[ocefpaf]

@Conengmo I'll take a look at the failures later but it seems that only Python 2.7 is affected and, b/c this feature is not really ready for prime time, I'm OK making it Python 3 only. (We already agreed that the next release is the last one for Python 2.7 anyway.)

Spoke too soon. All the failures were related to new pytest version. This one is good to go now.

[ocefpaf]

Merging this to b/c of the Travis-CI fixes in the last commit are needed for a new release.