Switch to context from the standard lib

[alicebob]

golang.org/x/net/context has moved into the standard library since Go 1.7 (1.8 is the current release). Switching to that package means projects using gopher-lua don't have to import the golang.org/x/net/context package anymore.

[yuin]

GopherLua currently supports >=Go1.6.
I'll do this at Go1.9 release.

golang.org/x/net/context has a fallback mechanism. If you use >=Go1.7, golang.org/x/net/context will use builtin context package as its implementation.

[alicebob]

ok, thanks!