Relax angular dependency to support angularjs 1.7

[sarod]

md-pickers should support having angular 1.7 as a dependency.

Today package.json limits usage to 1.5 or 1.6

[dpoetzsch]

As we converted our own project to vue, I did not touch this repo for a while. I am not sure if increasing the angular version might break anything (it used to in the past if I remember correctly).

But if someone wants to try it out and create a PR I'm happy to review it.

[sarod]

I guess this could be fixed by #41