BLD: travis cache, point pip install at upstream repo

Author: y-p

ci/prep_ccache.sh

@@ -3,7 +3,7 @@
 if [ "$IRON_TOKEN" ]; then
     sudo apt-get $APT_ARGS install ccache p7zip-full
     # iron_cache, pending py3 fixes upstream
-    pip install -I --allow-external --allow-insecure git+https://github.com/y-p/iron_cache_python.git
+    pip install -I --allow-external --allow-insecure git+https://github.com/iron-io/iron_cache_python.git@8a451c7d7e4d16e0c3bedffd0f280d5d9bd4fe59#egg=iron_cache
 
 
     python ci/ironcache/get.py