Use Node's built-in `path.isAbsolute` for compatibility with
Window's file paths.

Refactor versions helper to use lodash filters, instead of
nested if statements.

Remove the direct call to `tmp`'s manual cleanup function
as `tmp` adds a listener on process exit to call cleanup
on all created directories (that weren't created with the
`keep` option set to `true`).

This is considered a fix because under some conditions the cleanup
will fail on Windows, and because the cleanup is called manually
in the middle of a renovate cycle, any sporadic failure will fail
the entire renovate process.

Instead, defer the temporary directory cleanup until after renovate
has completed, where a failure won't de-rail the entire cycle.

(I have not been able to track down why `tmp` fails sometimes on
Windows with my various project setups.)

Must spawn `yarn` within a shell on Windows for the `yarn`
executable to be resolved correctly. Otherwise a ENOENT
error is received because the `yarn` command can't be found.

Spawning a comamnd in a shell on Linux should behave the same,
though there may be a small increase in time required to
spawn the process.

Closes #126 

* Update definitions.js

* Update docs

* Enhance logic for PR rebase check (GitHub)

Closes #119

* don’t set false

* update snapshots

* Update tests

* Fix yarn.lock detection

Closes #110, Closes #124

* Fix for tests :)

* Update dependency eslint to version 3.17.0

* Update dependency eslint to version 3.17.1

* Update dependency jest to version 19.0.0

* Update jest snapshot

Major refactor of worker code in order to add tests. Closes #99