Add ruby gems advisory importer

[sbs2001]

Signed-off-by: sbs2001 shivam.sandbhor@gmail.com

Fixes #86

[sbs2001]

I will soon write some tests for this importer

[haikoschol]

@sbs2001 Thanks for this PR! A few general nitpicks (other comments inline):

• please use snake_case instead of camelCase for variable names
• order imports alphabetically in three blocks: standard library, third-party libraries, modules inside vulnerablecode
• leave out parentheses in if and for if they are not necessary (i.e. not a multi-line statement)

[sbs2001]

@sbs2001 Thanks for this PR! A few general nitpicks (other comments inline):

* please use `snake_case` instead of `camelCase` for variable names

* order imports alphabetically in three blocks: standard library, third-party libraries, modules inside vulnerablecode

* leave out parentheses in `if` and `for` if they are not necessary (i.e. not a multi-line statement)

@haikoschol Thanks for the review! I have made the suggested changes.

[haikoschol]

Eventually we will have to include vulnerabilities without a CVE ID as well, since that is meant to be one of the defining features of vulnerablecode.