Add 'cache_file' parameter to 'deptrac' task.

[TravisCarden]

Q	A
Branch	master
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	no
Documented?	yes
Fixed tickets

Deptrac provides a --cache-file option to control where its cache file will be stored. This PR adds support for it with a new grumphp.tasks.deptrac.cache_file option in grumphp.yml, i.e.:

grumphp:
    tasks:
        deptrac:
            cache_file: ~

I updated the corresponding unit test. I didn't see any functional tests, but I manually tested it and it worked great.