Enable rustfmt checking locally and on Travis-ci about rav1e HOT 4 CLOSED

Current status: #43

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@mbebenita , @smarter
After one year later, "cargo fmt" still not move some code style from nightly to stable.
Currently, many codes do not have the right style.
How about we accept reality, use stable "cargo fmt" and enforce it in pre-commit hook and travis?
#1259 will enforce this

thanks

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The main sticking point is fn_args_layout, which is still not stable. We can either live with the default for this option or continue to not enforce rustfmt.

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Currently most of the options we use are stable on the beta branch of rustfmt.

There are three left-over unstable options:

• comment_width: this one actually has no effect because we don't set wrap_comments;
• match_arm_blocks: I'd argue it doesn't really make much difference and can be dropped;
• ignore: this can be dropped with the reformat.

I'll make a PR to drop these options (and also a couple of other ones that are set to the default values). After that, there are two options:

• run and set up the beta rustfmt right before the crate release;
• wait until the current beta rustfmt reaches stable (15th Aug 2019).

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