Fix BlockManagerSuite#after
Merge pull request #352 from markhamstra/oldArch

Don't leave os.arch unset after BlockManagerSuite

Recent SparkConf changes meant that BlockManagerSuite was now leaving the os.arch System.property unset.  That's a problem for any subsequent tests that rely upon having a valid os.arch.  This is true for CompressionCodecSuite in the usual maven build test order, even though it isn't usually true for the sbt build.