maint: integration with Travis CI system.

* .travis.yml: configure, build and test with each of Lua 5.1,
Lua 5.2 and luajit.
* GNUmakefile (luarocks-config.lua): Set interpreter environment
variables for luarocks invocations.
* mkrockspecs.lua: Invoke luarocks via user LUAROCKS setting.
* stdlib.rockspec.in (build.build_command): Pass LUA as a
configure precious variable so that recent ax-lua.m4 uses it as
passed instead of running the hardcoded search for a lua binary.
* README: Add travis build status badge markup.  Plus some
indentation corrections for correct markdown code rendering.