The solution I found was to add the following environment variables to spark-env.sh.  The first 2 lines make spark-shell able to read snappy files from when run in local mode and the third makes it possible for spark-shell to read snappy files when in yarn mode.

export JAVA_LIBRARY_PATH=$JAVA_LIBRARY_PATH:/usr/lib/hadoop/lib/native
export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:/usr/lib/hadoop/lib/native
export SPARK_YARN_USER_ENV="JAVA_LIBRARY_PATH=$JAVA_LIBRARY_PATH,LD_LIBRARY_PATH=$LD_LIBRARY_PATH"