Update Clippy version being used in TravisCI config file

[biskit1]

Update the version of clippy being used as per conversation here.
Lines with rustup component add clippy-preview (like the one shown below) should be changed to rustup component add clippy as per the Clippy docs.

supernova/.travis.yml

Line 41 in 66f5bc5

- rustup component add clippy-preview

[seanprashad]

This would be a good first bug - would you be available to mentor for it @biskit1?

[biskit1]

Sure, or I can do it.

[biskit1]

@0xazure should I add this to PR #39, or wait until that gets merged and then fix this?

[0xazure]

@biskit1 you can add this to #39 since it introduces rustfmt, or if you want to do it as a separate pull request that's fine too!