Go compiler updates

The build servers have been updated to Go 1.7. The dev builds linked above are now built with that compiler, as will the next release.

At the same time, the minimum specs for compiling Syncthing have been bumped to require at minimum Go 1.5.

There is currently a problem with the Windows build, and builder.

Just a heads up, building on Windows now requires Go 1.7 or later.