lua: Use lua_pushinteger for integer values in Lua 5.3+

[tomoe-mami]

In Lua 5.3+, when displaying values from Weechat functions that return integer the output will have .0 appended. This is because lua_pushnumber since 5.3 only pushes float value and float to string conversion now keeps the fractional part.

To reproduce, build Weechat with Lua 5.3+ and load this simple script. It should print the current buffer number followed by .0.

This PR replaces lua_pushnumber with lua_pushinteger to fix this issue.

[flashcode]

Thanks!