Fix slave.DEPS URL in launch_slaves.py
Fix slave.DEPS URL in launch_slaves.py

TBR=rmistry
BUG=skia:

Review URL: https://codereview.chromium.org/645573002

Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,Tiger66639/skia-buildbot,google/skia-buildbot,google/skia-buildbot