gears.surface: Add a version check for lgi

The cairo bindings are quite new in lgi and thus we depend on a quite new
version of lgi. Older versions will fail with lots of weird errors, e.g.:

lib/gears/color.lua:61:attempt to call field 'create_rgba' (a nil value)

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/surface.lua.in

@@ -9,6 +9,10 @@ local type = type
 local capi = { awesome = awesome }
 local cairo = require("lgi").cairo
 
+if tonumber(require("lgi.version")) <= 0.5 then
+    error("lgi too old, need at least version 0.7 (not yet released?) or recent git")
+end
+
 module("gears.surface")
 
 --- Try to convert the argument into an lgi cairo surface.