Remove System.register from UMD header

[rbuckton]

This PR removes System.register from the UMD header of tslib.js in favor of System's built in module format detection. This fixes an issue with how tslib.js loads when running inside of JSPM/SystemJS.

Fixes: microsoft/TypeScript#12887

[rbuckton]

@DanielRosenwasser can you publish an update to npm?

[aluanhaddad]

@rbuckton Just FYI, the global System is currently slated to be removed so as to not conflict with the System global proposed in TC39.
The current recommendation is to use the SystemJS global. This shouldn't affect module emit, at least for the time being, as a wrapper providing both names is currently injected.

You can see SystemJS rename to window.SystemJS over window.System for details.

Thanks! ❤️