[SPARK-7470] [SQL] Spark shell SQLContext crashes without hive

This only happens if you have `SPARK_PREPEND_CLASSES` set. Then I built it with `build/sbt clean assembly compile` and just ran it with `bin/spark-shell`.
```
...
15/05/07 17:07:30 INFO EventLoggingListener: Logging events to file:/tmp/spark-events/local-1431043649919
15/05/07 17:07:30 INFO SparkILoop: Created spark context..
Spark context available as sc.
java.lang.NoClassDefFoundError: org/apache/hadoop/hive/conf/HiveConf
	at java.lang.Class.getDeclaredConstructors0(Native Method)
	at java.lang.Class.privateGetDeclaredConstructors(Class.java:2493)
	at java.lang.Class.getConstructor0(Class.java:2803)
...
Caused by: java.lang.ClassNotFoundException: org.apache.hadoop.hive.conf.HiveConf
	at java.net.URLClassLoader$1.run(URLClassLoader.java:366)
	at java.net.URLClassLoader$1.run(URLClassLoader.java:355)
	at java.security.AccessController.doPrivileged(Native Method)
	at java.net.URLClassLoader.findClass(URLClassLoader.java:354)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:425)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:308)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:358)
	... 52 more

<console>:10: error: not found: value sqlContext
       import sqlContext.implicits._
              ^
<console>:10: error: not found: value sqlContext
       import sqlContext.sql
              ^
```
yhuai marmbrus

Author: Andrew Or <andrew@databricks.com>

Closes #5997 from andrewor14/sql-shell-crash and squashes the following commits:

61147e6 [Andrew Or] Also expect NoClassDefFoundError