Lua version agnostic

[pablomayobre]

As discussed in the forums, love-release script doesn't depend on the Lua version itself, it would be better to target any Lua version equal or greater than 5.1.

This would mean:

• Changing the rockspec
• Adding Lua 5.2 and Lua 5.3 to the Travis test matrix
• Changing the Luacheck --std flag to max
• Ensure that the other dependencies all work right in all the Lua versions
• Make lua-zip compatible with all the Lua versions

Also currently the tests are empty... So that is something that should be completed too

• Writing tests

And lastly but not least, working with different Lua versions means that conf.lua will be executed in versions different to the one used in LÖVE. This should be taken into account (to begin with, we could spit a warning... later on we should move to a new config file as discussed in #40 and #27 )

• Warning for conf.lua / New configuration file