BUG: Fixes BcolzMinuteBarMetadata to read the version correctly (#1425)

We were mistakenly using the minute_per_day field.

We now expose from the metadata object the version from which the
metadata was read. This allows a new test that verifies the version is
read correctly.

tests/data/test_minute_bars.py

@@ -36,6 +36,7 @@ from pandas import (
 )
 
 from zipline.data.minute_bars import (
+    BcolzMinuteBarMetadata,
     BcolzMinuteBarWriter,
     BcolzMinuteBarReader,
     BcolzMinuteOverlappingData,
@@ -88,6 +89,13 @@ class BcolzMinuteBarTestCase(WithTradingCalendars,
         )
         self.reader = BcolzMinuteBarReader(self.dest)
 
+    def test_version(self):
+        metadata = self.reader._get_metadata()
+        self.assertEquals(
+            metadata.version,
+            BcolzMinuteBarMetadata.FORMAT_VERSION,
+        )
+
     def test_write_one_ohlcv(self):
         minute = self.market_opens[self.test_calendar_start]
         sid = 1