Fix Travis CI failure of multiple_merges_during_fold_test

The environment that Travis runs test under unfortunately does not
immediately pick up the expected number of files to merge for the
different merge operations in the multiple_merges_during_fold_test.
This commit changes the test to re-try merging until the expected
result is seen (or eventually have the test timeout).
Merge branch '1.6'
Add ?IMPLIES(length(Ops) > 1, ...) to avoid legit model ambiguity
in cases where the op list is too short to make a reasonable &
correct guess of behavior.

Use eqc:testing_time/2 for test length & EUnit timeout at 2x
testing time.
Don't always return 'true', and re-indent the catch clause
Move include_lib for EUnit inside -ifdef(TEST)
Fix EUnit timeout mismatch in bitcask_qc_fsm.erl
Merge pull request #74 from basho/jdb-fix-travis-failure

Fix Travis CI failure of multiple_merges_during_fold_test

After many intermittent timing failures over in TravisCI land, we have this:

    BUILD PASSED: [bitcask jdb-fix-travis-failure:771c92d] by Scott Lystig Fritchie https://travis-ci.org/basho/bitcask/builds/5266676
Merge remote-tracking branch 'origin/1.6'

The 1.6 branch has diverged from master ... merging in recent mods.
Merge branch 'slf-bitcask_qc_expiry-intermittent-failures'
Fix race with deleting stale input files from merge, h/t @evanmcc

Evan found a failing QuickCheck + PULSE test case that causes a
fold to fail when racing with a merge.  The race appears to be
very difficult to trigger via QC + PULSE, alas, even when
tweaking the operation weights to get more puts + folds + merge
operations to happen on average.

My proposed fix is to move the call to `purge_setuid_files/1`
to the `init_keydir()` function.  `init_keydir()` knows if
we're opening the keydir for the first time (`not_ready` or
not), so purge the stale setuid files only in the `not_ready`
case.
Merge pull request #86 from basho/slf-purge-stale-file-race

Fix race with deleting stale input files from merge, h/t @evanmcc
Roll bitcask version 1.6.1
Merge tag '1.6.1' into 1.6

Tag bitcask 1.6.1