BUG: Fix price caching for tickers with '/' char

On Ubuntu (assume this is true for all posix) tickers containing a slash char ("CRD/A", "BRK/A", both valid tickers with yahoo api accessible timeseries) lead to a path error in loader.py line 286.

zipline/data/loader.py

@@ -272,8 +272,9 @@ must specify stocks or indexes"""
     if stocks is not None:
         for stock in stocks:
             print(stock)
+            stock_pathsafe = stock.replace(os.path.sep, '--')
             cache_filename = "{stock}-{start}-{end}.csv".format(
-                stock=stock,
+                stock=stock_pathsafe,
                 start=start,
                 end=end).replace(':', '-')
             cache_filepath = get_cache_filepath(cache_filename)