doc: update deps in build-unix.md after libevent

Add libevent, change usage of libssl from "secure communication" to
"crypto" that's more accurate after RPC SSL support removed.

Author: laanwj

doc/build-unix.md

@@ -31,8 +31,9 @@ These dependencies are required:
 
  Library     | Purpose          | Description
  ------------|------------------|----------------------
- libssl      | SSL Support      | Secure communications
- libboost    | Boost            | C++ Library
+ libssl      | Crypto           | Random Number Generation, Elliptic Curve Cryptography
+ libboost    | Utility          | Library for threading, data structures, etc
+ libevent    | Networking       | OS independent asynchronous networking
 
 Optional dependencies:
 
@@ -57,7 +58,7 @@ Dependency Build Instructions: Ubuntu & Debian
 ----------------------------------------------
 Build requirements:
 
-	sudo apt-get install build-essential libtool autotools-dev autoconf pkg-config libssl-dev
+	sudo apt-get install build-essential libtool autotools-dev autoconf pkg-config libssl-dev libevent-dev
 
 For Ubuntu 12.04 and later or Debian 7 and later libboost-all-dev has to be installed: