Support CVSS Version 3.1

[Fubinator]

Currently, it is only possible to use CVSS 3.0 vectors. In the future, I think it is a good idea to support Version 3.1 of the CVSS. The metrics of version 3.1 and 3.0 are identical, but the calculation is different.
When #8 is implemented, we can dynamically check which version the vector has and run the calculations based on that. When no version is specified we should use version 3.0, because it seems to be the most common version at the moment.

This issue could be split into multiple parts. I'm not exactly sure, yet.

[Fubinator]

After some thinking I came to the conclusion that maybe we should split the cvss.js file first.

1. Functions that are version-independent utility functions should be written to a file. (e.g. parseVectorObjectToString, isVectorValid)
2. Version dependent functions for calculation etc. should also be in a separate file.(e.g. calculateISC, calculateExploitability)

The more we split the version dependent functions into a separate file we can just add other files for other versions and determine with #8 which functions to use.

[Fubinator]

After investigation, it's seems like we need to implement exactly two things:

1. A roundUp ffunction which takes care of IEEE 754 floating point math ref1 ref2. All calculations for version 3.1 need to use this function. The function itself will be implemented with Version 3.1 compliant roundUp function #54.
2. The ModifiedImpact formula for the environmental score has changed. ref