retry conda environment install on Travis

[tomkinsc]

installation of conda packages on Travis often fails due to transient web request errors. This wraps the install command in a retry block to attempt the install up to 3 times.

[notestaff]

maybe better to disable error-checking locally, for commands allowed to fail, rather than for the whole script

[yesimon]

It seems like conda network commands are incredibly flaky lately, like 1/3 gateway errors?

[notestaff]

yes, so @tomkinsc 's fix would be great. though ideally conda itself would have a 'retries' setting.

[notestaff]

it seems conda has a built-in config setting for retries:

$ conda config --describe remote_max_retries

# remote_max_retries (int)

# The maximum number of retries each HTTP connection should attempt.

remote_max_retries: 3

[tomkinsc]

Thanks, @notestaff, nice find. It looks like conda does indeed retry 3 times by default, and we're still seeing network errors. I guess I'll try raising the retry count...
Looking at the rest of the config, we can also try turning the knobs remote_read_timeout_secs and remote_connect_timeout_secs if it seems like we're seeing timeout failures.

[tomkinsc]

Or maybe (some of) the network-related error messages are actually SSL/cert -elated?

• conda 4.5.0-py36_0 package gives error: CondaHTTPError: HTTP 000 CONNECTION FAILED for url  conda/conda#7150
• CondaHTTPError: HTTP 000 Connection Failed during conda install with new 2018.12 Anaconda3 Windows install conda/conda#8046
  I'll try adding an SSL/cert update to our conda install script.