[SPARK-6371] [build] Update version to 1.4.0-SNAPSHOT.

Author: Marcelo Vanzin <vanzin@cloudera.com>

Closes #5056 from vanzin/SPARK-6371 and squashes the following commits:

63220df [Marcelo Vanzin] Merge branch 'master' into SPARK-6371
6506f75 [Marcelo Vanzin] Use more fine-grained exclusion.
178ba71 [Marcelo Vanzin] Oops.
75b2375 [Marcelo Vanzin] Exclude VertexRDD in MiMA.
a45a62c [Marcelo Vanzin] Work around MIMA warning.
1d8a670 [Marcelo Vanzin] Re-group jetty exclusion.
0e8e909 [Marcelo Vanzin] Ignore ml, don't ignore graphx.
cef4603 [Marcelo Vanzin] Indentation.
296cf82 [Marcelo Vanzin] [SPARK-6371] [build] Update version to 1.4.0-SNAPSHOT.