remove old geth version requirement for whisper

Author: iurimatias

docs/installation.rst

@@ -1,7 +1,7 @@
 Installation
 ============
 
-Requirements: geth (1.6.5 or higher recommended, 1.6.0 or lower for whisper support), node (6.9.1 or higher is recommended) and npm
+Requirements: geth (1.6.5 or higher recommended), node (6.9.1 or higher is recommended) and npm
 serpent (develop) if using contracts with Serpent, testrpc (3.0 or higher) 
 if using the simulator or the test functionality. Further: depending on
 the dapp stack you choose: `IPFS <https://ipfs.io/>`__