SPARK-1851. Upgrade Avro dependency to 1.7.6 so Spark can read Avro file...

...s

Author: Sandy Ryza <sandy@cloudera.com>

Closes #795 from sryza/sandy-spark-1851 and squashes the following commits:

79c8227 [Sandy Ryza] SPARK-1851. Upgrade Avro dependency to 1.7.6 so Spark can read Avro files
(cherry picked from commit 08e7606a964e3d1ac1d565f33651ff0035c75044)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>