Fix broken book publishing on Travis

[yerke]

The books were not published on https://docs.rust-embedded.org/ since March 16 2020. Last successful build here, first broken build here. Here you can see the long list of failures.

I took the changes from https://github.com/rust-embedded/discovery/blob/d42012eedb4216ae8b613c3fdfdb37f85f01656f/ci/install.sh .

For proof that the changes fix the issue see my fork in https://github.com/yerke/docs, which was successfully built with Travis in here. You can see built artifacts at https://github.com/yerke/yerke.github.io/tree/gh-pages and https://yerke.github.io .

Closes #8, rust-embedded/wg#457, rust-embedded/discovery#253

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @thejpster (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[yerke]

And of course after I spent the whole day chasing this, it turns out that this is the same as #21 :(

[therealprof]

@yerke Not sure I follow. Does this PR fix the problem?

[yerke]

It does, yes, @therealprof. You can check https://travis-ci.com/github/yerke/docs/builds/174282136 for successful Travis build and https://yerke.github.io/ as example of built books, as I mentioned in the description.

[therealprof]

Thank you, then let's go with this.

[therealprof]

bors r+

[yerke]

@therealprof Thank you for being so responsive.

[bors]

Build succeeded:

• continuous-integration/travis-ci/push

[therealprof]

Thank you for putting time into this. Much appreciated!