Broken lgi version checking

Currently it takes `require('lgi.version')`, converts it blindly to
number and compares with another value.
I am using lgi v0.6.1 and that is not convertable to number value,
thus tonumber() returns nil and crashes Awesome.
The fix is to take major and minor numbers only ignoring the rest.

Signed-off-by: Arvydas Sidorenko <asido4@gmail.com>
Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/surface.lua.in

@@ -9,7 +9,7 @@ local type = type
 local capi = { awesome = awesome }
 local cairo = require("lgi").cairo
 
-if tonumber(require("lgi.version")) <= 0.5 then
+if tonumber(string.match(require('lgi.version'), '(%d%.%d)')) <= 0.5 then
     error("lgi too old, need at least version 0.7 (not yet released?) or recent git")
 end