Suggest workarounds for partitionBy in Spark 1.0.0 due to SPARK-1931

The Graph.partitionBy operator allows users to choose the graph partitioning strategy, but due to SPARK-1931, this method is broken in Spark 1.0.0. This PR updates the GraphX docs for Spark 1.0.0 to encourage users to build the latest version of Spark from the master branch, which contains a fix. Alternatively, it suggests a workaround involving partitioning the edges before constructing the graph.