dep broken because of moved fsnotify package

[aaron7]

Solving failure: No versions of gopkg.in/fsnotify.v1 met constraints:
        v1.4.2: unable to update checked out version: fatal: reference is not a tree: 629574ca2a5df945712d3079857300b5e4da0236
: command failed: [git checkout 629574ca2a5df945712d3079857300b5e4da0236]: exit status 128
make: *** [update-vendor] Error 1

Merged prometheus/prometheus#3995 to fix upstream.

Will now update in cortex.

[aaron7]

Reverted #763

[bboreham]

Closed by #802