Fix# 1982: Loading YAML Safely

[krtkvrm]

Fixes: #1982

Updated PyYaml to 4.2b1

Prior to #74, yaml.load could run arbitrary python when parsing yaml, by design. #74 changed this so that by default, load didnt run python, and instead was an alias to safe_load. In this release, this was reverted, which means it's now a known security issue, hence the release of a security notice (CVE-2017-18342).
From : Security fixe reported by github #243

Directly Updated to 4.2b1 as 4.1 was retracted

Also, replaced yaml.load() with yaml.safe_load() in apps/challenges/views.py#L566

In PyYAML before 4.1, the yaml.load() API could execute arbitrary code. In other words, yaml.safe_load is not used.
From : National Vulnerability Database : CVE-2017-18342

Edit: This PyYAML 4.2 Release Plan acts on reverting the PR : Make pyyaml safe by default. #74 which is still not closed.

Make PR to revert #74 ( #194 )

This, the commit Loading YAML Safely in this PR deals with this.

[krtkvrm]

@RishabhJain2018 @deshraj Please Review

[RishabhJain2018]

@vkartik97 This looks good though! Can you please add a link to the PR or the documentation in the description where it says PyYAML 4 loads the data safely by default.

[krtkvrm]

@RishabhJain2018

This PyYAML 4.2 Release Plan acts on reverting the PR : Make pyyaml safe by default. #74 which is still not closed.

Make PR to revert #74 ( #194 )

This, the commit Loading YAML Safely in this PR deals with this.

[RishabhJain2018]

LGTM :)