Use GRADLE_USER_HOME when available to init cache

[TamasBarta]

Gradle's home directory's location is configurable by the GRADLE_USER_HOME environment variable. This change follows that variable for the completion cache, so users can keep their home directory clean.

[TamasBarta]

Hey, sorry about the duplication, I just found #100. #96 is similar, but a bit more verbose. Please close this one, and merge one of the others, or merge this one, as it passes the checks, so it needs no modifications.

[lestephane]

3 PRs, no actual issue, makes it hard to +1 in the 'correct place'. I +1 this one for no other reason than it can be merged, and has the most recent activity.

[alllex]

Fixed in #130