DOC: cleaned references to pandas v0.15 and v0.16 in docs
Last round of cleanup of references to older pandas versions (here v0.15 and v0.16).

I'm not intending to go further up:  v0.16.2 was released in june 2015, and allowing up to 2 years old references to be seen in the docs themselves is reasonable IMO. (but other may have a different view, so if there's a "demand" I could change the cut-off point, its not so much work).

See also #17420 and #17404.

I also removed a reference to python 3.4  in the ``install.rst``. In  the same document, I also "updated" the test section to show python3.6 (just a (very) minor signal that pandas is python 3 positive).
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=h1) Report
> Merging [#17442](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d4577911c750f2f48f760ce451d413116bed72da?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/17442/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17442      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49587    49587              
==========================================
- Hits        45203    45194       -9     
- Misses       4384     4393       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <ø> (ø)` | :arrow_up: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=footer). Last update [d457791...f207219](https://codecov.io/gh/pandas-dev/pandas/pull/17442?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 pls also add a whatsnew entry on Other Enhancements that docs are now updated to remove older references (and you can list the issue / PR's that were involved). ping @jreback  thanks @topper-123 very nice series of changes!

when docs are built (could be a while), see http://pandas-docs.github.io/pandas-docs-travis/

and if anything needs corrections pls push a PR. Ok, I'll go look in a few days.

Yeah, these 3 commits have a net reduction in line count of ca. 330 lines, that's quite a bit of deadweight removed :-)