[slave]: Add quote to package version range

[stcheng]

Without quote, the command will ignore the version and
regard the ">" as a file redirection symbol and redirect
all the output to a file named the version range.

Signed-off-by: Shu0T1an ChenG shuche@microsoft.com

[stcheng]

Found interesting stuffs in the slave docker

root@4b14a44af963:/# ls -al
...
-rw-r--r--  34 root root  773 Jul 12 06:46 =2.10
-rw-r--r--  29 root root   72 Jul 12 06:47 =5.1
...
root@4b14a44af963:/# cat \=2.10
Collecting jinja2
  Downloading https://files.pythonhosted.org/packages/1d/e7/fd8b501e7a6dfe492a433deb7b9d833d39ca74916fa8bc63dd1a4947a671/Jinja2-2.10.1-py2.py3-none-any.whl (124kB)
Collecting MarkupSafe>=0.23 (from jinja2)
  Downloading https://files.pythonhosted.org/packages/fb/40/f3adb7cf24a8012813c5edb20329eb22d5d8e2a0ecf73d21d6b85865da11/MarkupSafe-1.1.1-cp27-cp27mu-manylinux1_x86_64.whl
Installing collected packages: MarkupSafe, jinja2
  Found existing installation: MarkupSafe 0.23
    Not uninstalling markupsafe at /usr/lib/python2.7/dist-packages, outside environment /usr
  Found existing installation: Jinja2 2.8
    Not uninstalling jinja2 at /usr/lib/python2.7/dist-packages, outside environment /usr
Successfully installed MarkupSafe-1.1.1 jinja2-2.10.1
root@4b14a44af963:/# cat \=5.1
Requirement already satisfied: PyYAML in /usr/lib/python3/dist-packages

[lguohan]

interesting.