Fix/undefined versions

[morreman]

Since silentSyncLoadJsonFile() can return undefined we should(?) nullguard the versions variable.
From what I understand it should be ok for the versions variable to be undefined.

// It's okay for it not to exist

We have some repos whos travis builds is failing on this

Failed to init systems: TypeError: Cannot read property 'files' of undefined

For example when running mocha on Jalla

at AssetVersions._loadAssetDefinitions (/home/runner/work/jalla/jalla/node_modules/@hdsydsvenskan/asset-versions/index.js:55:36)

// Side-note the build-script won't finnish if one is missing the dist/rev dist/bare folders. I changed the gitIgnore but you might want to do it a different way? I see that its been failaing on this for a while so I revert my ignore edit

[voxpelli]

@morreman I made a slightly different and deeper fix than this, ensuring that the data read is actually valid all the way through. Thanks for pointing it all out!