#1250

The global plotly library object was renamed from 'Plotly' to '_Plotly'
in plotly.py 3.4.0 to avoid a naming conflict when a Jupyter Notebook
heading contained the text 'Plotly'

See:
plotly/plotly.py#816
plotly/plotly.py#1250

* 'k' is not a valid plotly color string, use 'black' instead
's' is not a valid plotly symbol string, use 'circle' instead
'plasma' is not a valid plotly colorscale, use 'viridis' instead

* Rename global plotly library object in JavaScript

The global plotly library object was renamed from 'Plotly' to '_Plotly'
in plotly.py 3.4.0 to avoid a naming conflict when a Jupyter Notebook
heading contained the text 'Plotly'

See:
plotly/plotly.py#816
plotly/plotly.py#1250

* Replace use of deprecated graph object classes

Graph objects are now structured hierarchically, so go.Marker was
deprecated in favor of go.layout.Marker.  The use of these classes
is fully optional, and they can be replaced by plain dict instances in
cases where local validation, tab completions, and docstrings aren't
needed.

* The figure.data property is now a tuple rather than a list, so it
cannot be mutated in place. Instead, the add_traces method is used
to append additional traces to the figure.

* Object arrays (like annotations) are now stored as tuples
rather than lists so they cannot be extended in place.
The += operator can be used instead to replace the object array with
an extended version of itself.

* Graph objects are no longer dict subclasses, they are wrappers around
dict instances with some dict-like methods. The to_plotly_json method
is used to convert a graph object into a Python dict.

* Update plotly in holoviews environment to version 3.4 from the plotly channel

* Validate required plotly version

* Replace deprecated append_trace method call with add_trace

* Subplot references ending in 1 (e.g. x1) now have the 1 removed (e.g. x)

* Axis ranges are now returned as tuples not lists

* Remove unused import causing flake8 failure

* When exporting to standalone html the plotly library is loaded as Plotly
but when loaded into the notebook it is loaded as _Plotly.

(before this commit figures rendered properly in the jupyter notebook
but an error was raised when output to an HTML file that _Plotly is
not defined)

* Run conda env update with travis_wait command to (hopefully) avoid CI timeout.

See https://docs.travis-ci.com/user/common-build-problems/#build-times-out-because-no-output-was-received

Prior intermittent CI error:
```
...
$ conda env update -n holoviews -q -f environment.yml
Solving environment: ...working... done
Preparing transaction: ...working... done
Verifying transaction: ...working... done
Executing transaction: ...working... dbus post-link :: /etc/machine-id not found ..
dbus post-link :: .. using /proc/sys/kernel/random/boot_id
No output has been received in the last 10m0s, this potentially indicates a stalled build or something wrong with the build itself.
```

* remove stray print statement

…"Plotly" (plotly#1250)

This PR works around the issue where a Jupyter Notebook heading
named "Plotly" clashes with the window.Plotly variable where we store
the plotly.js module.

The workaround is to name this variable the much less likely _Plotly