Invalidate cached dependencies on dependency updates

[cameel]

Depends on #756.
Related to #757 and #758.

Our caching of dependencies is based only on the content of package.json. This means that CI is running with versions that were most recent when it was last updated. This PR adds the output of npm outdated to the cache key to invalidate the cache whenever new versions appear.

Note that this is still not foolproof:

• Cache will get invalidated even if we would not install the new version. E.g. if we depend on x@^1.0.0, the release of [email protected] will still trigger it, even though we'd only pull the last version of the 1.x series.
• We might miss an update if the newly released version is not the latest one. E.g. if we depend on x@^1.0.0 and [email protected] already exists, [email protected] will not trigger it.

Despite these flaws, this is probably good enough and better than the current situation. We update package.json on every release anyway so we already do notice such breakage eventually. This PR would just make it much more likely that it won't remain undiscovered until the very day of the next release.

This becomes more important if we decide to merge #758, because that change would make solc-js automatically switch to new dependencies even if their version indicates they're breaking. We'd need to more actively react by adding an upper limit when that happens.

[coveralls]

coverage: 84.537%. remained the same
when pulling 9a210ab on invalidate-cache-on-dependency-updates
into 95d3a5f on master.