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 branch-1.0, which contains a fix. Alternatively, it suggests a workaround involving partitioning the edges before constructing the graph.