Fix IllegalAccessError in MrContext when running in local mode

Reflection seems to work fine.

/cc @markhibberd @raronson (and @etorreborre might be interested)

I've cherry-picked this to the oldest possible commit so I can merge into something later for a temporary release for our regression tests. (I'm thinking 8c5a62b which is where I disabled snappy compression, but just before the dictionary changes)

```
[WARN] LocalJobRunner - job_local1120324947_0001 <java.lang.Exception: java.lang.IllegalAccessError: tried to access class org.apache.hadoop.mapreduce.lib.input.TaggedInputSplit from class org.apache.hadoop.mapreduce.lib.input.ProxiedInpu
tSplit$>java.lang.Exception: java.lang.IllegalAccessError: tried to access class org.apache.hadoop.mapreduce.lib.input.TaggedInputSplit from class org.apache.hadoop.mapreduce.lib.input.ProxiedInputSplit$
        at org.apache.hadoop.mapred.LocalJobRunner$Job.runTasks(LocalJobRunner.java:462)
        at org.apache.hadoop.mapred.LocalJobRunner$Job.run(LocalJobRunner.java:522)
Caused by: java.lang.IllegalAccessError: tried to access class org.apache.hadoop.mapreduce.lib.input.TaggedInputSplit from class org.apache.hadoop.mapreduce.lib.input.ProxiedInputSplit$
        at org.apache.hadoop.mapreduce.lib.input.ProxiedInputSplit$.fromInputSplit(ProxyTaggedInputSplit.scala:24)
        at com.ambiata.ivory.mr.MrContext$.getSplitPath(Context.scala:62)
        at com.ambiata.ivory.ingest.IngestMapper.setup(mr.scala:206)
        at org.apache.hadoop.mapreduce.Mapper.run(Mapper.java:142)
```
> Can this be just ...

Whoops. Of course, forgot about that method.