Ensure html_root_url is kept in sync

[mgeisler]

This adds two new unit tests: one that checks that the html_root_url
attribute always uses the current crate version number, and one that
checks that the TOML code blocks in the README use the correct version
number.

This is implemented by my version-sync crate -- I hope it's useful!

Fixes #613.

[mgeisler]

This might be more complicated than I hoped since version-sync doesn't support Rust 1.14 (version 1.15 is the oldest it can be compiled on).

[mgeisler]

Please let me know if there's any interest in the general approach — then I can see about removing the version-sync dependency on versions and targets where it doesn't work (remove with sed in the Travis build).

[carllerche]

Thanks for the PR.

I'd rather not add another dependency and instead deal with this by having a RELEASE doc and wait for this to get fixed up stream. But, again, thanks for the attempt :).

[twmb]

As an alternative, I've used

before_install: |
  version=$(grep "^version =" Cargo.toml | sed -r  's/^[^0-9]*([0-9]+.[0-9]+.[0-9]+).*/\1/') &&
  docs_version=$(grep -R 'docs\.rs' src | head -n 1 | sed -r 's/^[^0-9]*([0-9]+.[0-9]+.[0-9]+).*/\1/') &&
  test "$version" == "$docs_version"

in my .travis.yml in the past. This only catches the first mismatch, but it could be cleaned up to catch all mismatches. But, waiting for upstream fixes is 👍 as well.

[mgeisler]

@carllerche Sure, no problem. Note that it's only a new dev-dependency, so only people who run the MIO tests are affected by this.

I'll see about making another PR that at least fixes the outdated version number :-)

[carllerche]

@twmb I think a smoke test like that would be good.

@mgeisler Sure re: PR that fixes the version :)

[mgeisler]

I've submitted #760 to fix the version number.