[SPARK-8821] [EC2] Switched to binary mode for file reading

Otherwise the script will crash with

    - Downloading boto...
    Traceback (most recent call last):
      File "ec2/spark_ec2.py", line 148, in <module>
        setup_external_libs(external_libs)
      File "ec2/spark_ec2.py", line 128, in setup_external_libs
        if hashlib.md5(tar.read()).hexdigest() != lib["md5"]:
      File "/usr/lib/python3.4/codecs.py", line 319, in decode
        (result, consumed) = self._buffer_decode(data, self.errors, final)
    UnicodeDecodeError: 'utf-8' codec can't decode byte 0x8b in position 1: invalid start byte

In case of an utf8 env setting.

Author: Simon Hafner <hafnersimon@gmail.com>

Closes #7215 from reactormonk/branch-1.4 and squashes the following commits:

e86957a [Simon Hafner] [SPARK-8821] [EC2] Switched to binary mode

(cherry picked from commit 83a621a5)
Signed-off-by: Shivaram Venkataraman <shivaram@cs.berkeley.edu>