(limited) support for building w/ master

[sbinet]

Proposal

it would be great if one could build yægi using the Go gc compiler's next version.
right now, when using the compiler from master, ad compiling yægi, one gets:

$> go get -v ./...
package github.com/traefik/yaegi/cmd/yaegi
	imports github.com/traefik/yaegi/stdlib/syscall: build constraints exclude all Go files in /home/binet/work/gonum/src/github.com/containous/yaegi/stdlib/syscall
package github.com/traefik/yaegi/cmd/yaegi
	imports github.com/traefik/yaegi/stdlib/unsafe: build constraints exclude all Go files in /home/binet/work/gonum/src/github.com/containous/yaegi/stdlib/unsafe

the standard library doesn't insert that many new symbols from one release cycle to the next, so it seems to me it would be relatively cheap to just add the build constraints for n+1 whenever the Go release n is cut.

added benefit: earlier bug reports w/ the next Go release that yægi will have to support.

[mvertes]

With #943, the build tag on next Go version will be not added to wrappers, allowing to build yaegi using a Go devel version with support of stdlib. Note that go generate must not be run, as symbol extraction will fail with non stable Go versions.