[DOC] Suggest to upgrade pygit2 and deps

[vutny]

What does this PR do?

It convinces user to upgrade to latest pygit2 version and its dependencies in order to resolve issues with http[s]:// GitFS endpoints on RHEL 7. Mentioned issues in EPEL have been fixed, so the downgrading workaround no longer nesessary.
Confirmed with following versions:

Salt Version:
           Salt: 2016.11.8
 
Dependency Versions:
           cffi: 1.6.0
       cherrypy: unknown
       dateutil: 1.5
      docker-py: Not Installed
          gitdb: Not Installed
      gitpython: Not Installed
          ioflo: Not Installed
         Jinja2: 2.7.2
        libgit2: 0.24.6
        libnacl: Not Installed
       M2Crypto: 0.21.1
           Mako: Not Installed
   msgpack-pure: Not Installed
 msgpack-python: 0.4.8
   mysql-python: Not Installed
      pycparser: 2.14
       pycrypto: 2.6.1
   pycryptodome: Not Installed
         pygit2: 0.24.2
         Python: 2.7.5 (default, Nov 20 2015, 02:00:19)
   python-gnupg: 0.3.8
         PyYAML: 3.11
          PyZMQ: 15.3.0
           RAET: Not Installed
          smmap: Not Installed
        timelib: Not Installed
        Tornado: 4.2.1
            ZMQ: 4.1.4
 
System Versions:
           dist: centos 7.4.1708 Core
        machine: x86_64

Commits signed with GPG?

Yes.

[terminalmage]

Thanks!