[Minor] Update dependencies on README.md

[seojangho]

• We do not need REEF snapshot as we provide bundled Maven repository
• Running builds(./configure and make) with sudo is generally not a good idea
• Added PPA for Ubuntu xenial, for convenience.

[wonook]

Can you add these lines for Mac OSX as well?

$ brew tap homebrew/versions
$ brew install protobuf@2.5

[seojangho]

Thanks, @wonook. I've addressed your comments.