Adds Travis-Ci support and fixes the test with Lua 5.2

This PR adds integration with Travis-CI. It configures a test matrix that runs the test using Lua 5.1 and Lua 5.2 against Redis 2.4, 2.6 and 2.8

This exposed a bug in the test suite that is also fixed in this PR.

You can see the output here:
https://travis-ci.org/ignacio/redis-lua/builds/26586737