BUILD: travis: TMPDIR replacement.

BUILD: travis: TMPDIR replacement.

TMPDIR default value may be too long to create UNIX sockets for the stats
used during the reg tests. Indeed vtest builds its temporary working directory
${tmpdir} variable from TMPDIR variable, with /tmp as value if not already set.
This is the case on Linux contrary to OS X which sets TMPDIR with a too much long
value.

With this path we revert the part of 88c63a6 commit which tried to shorten this
TMPDIR value modifying script/run-regtests.sh. Unfortunately this was not
sufficient. Furthermore this patch force TMPDIR to /tmp value for all the OS'es.

Thank you to Tim Düsterhus and Ilya for having helped on this issue.