Added multiplatform CI testing

It was mentioned in comments [here](https://github.com/Olivine-Labs/luassert/pull/51) that it would be nice to set up Travis to run tests on several Lua versions. I added a small script for installing Lua and Luarocks and changed .travis.yml to run three times, for Lua 5.1.5, Lua 5.2.3 and LuaJIT 2.0.2. Tell me if something should be tweaked! 

[This is how it looks on travis-ci.org](https://travis-ci.org/mpeterv/luassert/builds/19987902)