CI using old cache of mdbook

[ehuss]

I'm not 100% certain without access to the Travis cache, but I believe that it is using an old version of mdbook (0.0.28) from the cache. This was noticed because Travis is not running mdbook-linkcheck, and the output in the Travis logs looks like the old version.

In the Travis logs, it looks like PRs are picking up this old cache from the master branch:

fetching master/cache-linux-trusty-e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855--cargo-stable.tgz
found cache

Due to the way ci/install.sh is written, it will skip if mdbook is already installed, but it does not check which version is already installed.

A quick short-term solution is to delete the travis caches.

I can make a PR to change the install.sh script to force a reinstall if there is a version mismatch. Alternatively, you can just manually delete the travis caches whenever you upgrade mdbook.

[mark-i-m]

I can make a PR to change the install.sh script to force a reinstall if there is a version mismatch.

Thanks @ehuss ! I would appreciate that :)

It explains #47 (comment)

[mark-i-m]

I just dropped the caches, and will restart the travis builds for outstanding PRs, but a permanent fix would be good.