Upgrade to lua 5.3 - Githubissues

OK, so travis is passing. I had to lock a bundler version in .travis.yml. Used the version in Gemfile.lock.

To summarize the changes:

Removed setenv/getenv (lua_getfenv and lua_setfenv were removed in 5.2)
Changes rbLua_get_global/rbLua_set_global to use lua_getglobal/lua_setglobal instead of getenv/setenv
Removed bootstrap_unpack. It was moved to table.unpack.
lua_objlen was deprecated, using lua_rawlen instead
lua_equal changed to lua_compare with LUA_OPEQ
Updated bootstrap_* to match lua/lbaselib.c
Couldn't find the source for inext, but had to upgrade it to remove luaL_checkint. Unsure if I did it right.
Added some basic tests to bootstrap_* functions. Could add tests for the other functions in a future PR.

whitequark / rlua