DOC: Updated sample CI image in contributing docs
The image of a green CI in https://pandas-docs.github.io/pandas-docs-travis/contributing.html#testing-with-continuous-integration became outdated (contains circleci and appveyor). Replaced by a current one with only travis and azure.

![pandas_ci_doc_screenshot](https://user-images.githubusercontent.com/10058240/50937863-8d87f880-146d-11e9-8a3b-6c32bcd25e36.png)


sure
merge on green # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=h1) Report
> Merging [#24692](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/decc8ce5601c32dc2313f0d2b1d611164262cae9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24692/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24692   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52310    52310           
=======================================
  Hits        48327    48327           
  Misses       3983     3983
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.06% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24692?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/24692?src=pr&el=footer). Last update [decc8ce...b03198e](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=h1) Report
> Merging [#24692](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/decc8ce5601c32dc2313f0d2b1d611164262cae9?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24692/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #24692   +/-   ##
=======================================
  Coverage   92.38%   92.38%           
=======================================
  Files         166      166           
  Lines       52310    52310           
=======================================
  Hits        48327    48327           
  Misses       3983     3983
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `43.06% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24692?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/24692?src=pr&el=footer). Last update [decc8ce...b03198e](https://codecov.io/gh/pandas-dev/pandas/pull/24692?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
