Spy calls log file is not thread-unique

[damienflament]

The file containing spy logs is unique for each example but not for each thread.

So, running Shellspec with multiple jobs (--jobs option with value superior to 1) cause some expectations to occurs on Spy logs from other thread.

travis-python/spec/helper.sh

Lines 11 to 12 in 3a07f8e

	FIXTURES_BASE=$SHELLSPEC_TMPBASE/fixtures
	mkdir "$FIXTURES_BASE" # FIXME This is not thread-safe !

travis-python/spec/support/doubles.sh

Line 76 in 3a07f8e

export SPY_CALLS_FILE="$SPY_DIR/$SHELLSPEC_EXAMPLE_ID.log"

Investigating on a Shellspec job identifier provided by an environment variable is required.