added explicit dependency for org.xerial.snappy 

I was have problem while building spark-jobserver due dependency resolve issue. Adding direct dependency to *org.xerial.snappy* solved this issue.

```
$ sbt compile
...
[info] Resolving com.sun.jersey#jersey-client;1.9 ...
[error] impossible to get artifacts when data has not been loaded. IvyNode = org.xerial.snappy#snappy-java;1.0.4.1
[info] Resolving com.sun.jersey#jersey-client;1.9 ...
[error] impossible to get artifacts when data has not been loaded. IvyNode = org.xerial.snappy#snappy-java;1.0.4.1
java.lang.IllegalStateException: impossible to get artifacts when data has not been loaded. IvyNode = org.xerial.snappy#snappy-java;1.0.4.1
...
[error] (job-server-tests/*:update) java.lang.IllegalStateException: impossible to get artifacts when data has not been loaded. IvyNode = org.xerial.snappy#snappy-java;1.0.4.1
[error] (job-server/*:update) java.lang.IllegalStateException: impossible to get artifacts when data has not been loaded. IvyNode = org.xerial.snappy#snappy-java;1.0.4.1
[error] Total time: 12 s, completed 2014-03-26 23:25:26
```