convert index alias to microseconds

[jfrolich]

Because the index name is now tied to a particular second, when testing a module that uses elasticsearch-elixir, the index name will collide.
Changed to microsecond.

[coveralls]

Coverage remained the same at 97.99% when pulling 3979520 on jfrolich:microseconds into 18f3c51 on infinitered:master.

[danielberkompas]

Thanks @jfrolich!