Another Hadoop native libraries warning fix.

By default 64bit Apache Hadoop binaries come with 32bit native libraries, which, unless compiled separately, cause warning header to be appended:

"OpenJDK 64-Bit Server VM warning: You have loaded library /usr/local/hadoop-2.3.0/lib/native/libhadoop.so.1.0.0 which might have disabled stack guard. The VM will try to fix the stack guard now.
It's highly recommended that you fix the library with 'execstack -c <libfile>', or link it with '-z noexecstack'."

>>> from luigi import hdfs
>>> cmd = ['hadoop', 'fs', '-count', '/cache']
>>> stdout = hdfs.call_check(cmd)
>>> print stdout
OpenJDK 64-Bit Server VM warning: You have loaded library /usr/local/hadoop-2.3.0/lib/native/libhadoop.so.1.0.0 which might have disabled stack guard. The VM will try to fix the stack guard now.
It's highly recommended that you fix the library with 'execstack -c <libfile>', or link it with '-z noexecstack'.
           1            1           50214873 /cache

This fix will get rid of this header.
Merge pull request #505 from liquid-sky/master

Another Hadoop native libraries warning fix.