Remove hardcoded call to /bin/sh in use-standalone-widevine.sh

[marcor]

Fix issues #9 and #11.

[GlowingScrewdriver]

I second this. When fetch-standalone-widevine.sh is executed with a shell other than bash (on Debian, for example, /bin/sh usually points to dash by default), the tests that use [[ fail, since [[ is a bash builtin

I can confirm that with dash as my default shell the script does not work; and that removing the hardcoded call indeed does make it work.