build: require gcc >= 5.3, boost >= 1.58, openssl >= 1.0.2

This effectively drops support for all versions of Ubuntu older than 16.04

Change-Id: Ie3ab7df9147e97f6467658a6399a4f9379f089c1
Refs: #4462
diff --git a/docs/INSTALL.rst b/docs/INSTALL.rst
index 019ed5b..0eaf52a 100644
--- a/docs/INSTALL.rst
+++ b/docs/INSTALL.rst
@@ -9,7 +9,6 @@
 ndn-cxx uses continuous integration and has been tested on the following
 platforms:
 
--  Ubuntu 14.04 (amd64)
 -  Ubuntu 16.04 (amd64, armhf, i386)
 -  Ubuntu 18.04 (amd64)
 -  OS X 10.11
@@ -30,11 +29,12 @@
 Required:
 ~~~~~~~~~
 
+-  GCC >= 5.3, or clang >= 3.5
 -  ``python2`` >= 2.7, or ``python3`` >= 3.4
 -  ``libsqlite3``
--  OpenSSL >= 1.0.1
+-  OpenSSL >= 1.0.2
 -  ``pkg-config``
--  Boost libraries >= 1.54
+-  Boost libraries >= 1.58
 -  OSX Security framework (on OSX/macOS platform only)
 
 Following are the detailed steps for each platform to install the compiler, all necessary