Move enforcement of crate level forbid and deny from compiler to CI

[prasannavl]

Feature Request

With the insights expressed by @Nemo157 here: #222 (comment), it eases development to have all the crate level enforcement done at CI, and instead default to warn for all code quality enforcement.

Surface area touched by: #220 #222 #194

Possible Implementation

• Switch all crate level forbid and deny into warn
• Enforce CI by adding --deny=warnings appropriately to .travis.yml

[Nemo157]

The second part of the implementation is already covered as part of #228, the clippy job will fail if any rustc or Clippy warnings are triggered.