[Doc] Target protobuf 2.6 in OS X build notes.

Homebrew now installs Protobuf version 3 by default, which doesn't currently compile. Install Protobuf 2.6.x from the versions tap instead.

Author: fanquake

doc/build-osx.md

@@ -16,7 +16,7 @@ Then install [Homebrew](http://brew.sh).
 Dependencies
 ----------------------
 
-    brew install automake berkeley-db4 libtool boost --c++11 miniupnpc openssl pkg-config protobuf --c++11 qt5 libevent
+    brew install automake berkeley-db4 libtool boost --c++11 miniupnpc openssl pkg-config homebrew/versions/protobuf260 --c++11 qt5 libevent
 
 NOTE: Building with Qt4 is still supported, however, could result in a broken UI. Building with Qt5 is recommended.