Bump mozjs_sys version to 0.115.7-0

[sagudev]

In #454 we got the need to have versioned mozjs-sys (even if unpublished) for artifact purposes so I proposed the following versioning for mozjs-sys (modeled after what we used before we give up on versioning):
0.{SM-major}.{SM-minor}-# where # is sequential number that is incremented for every C/C++ change in same SM version.

So we would start with 0.115.7-0 (we currently use SM 115.7) then for every C* change increment last number (0.115.7-1) until we move on to new SM 0.115.9-0 that would again start with new number.

CC @jdm @mrobinson @wusyong

[wusyong]

Maybe it can just use major version already? So it can be like this: {SM-major}.{SM-minor}.{#}, and the current version could be 115.7.1.

[sagudev]

Maybe it can just use major version already? So it can be like this: {SM-major}.{SM-minor}.{#}, and the current version could be 115.7.1.

Then # changes could not be breaking if we want to uphold semver. We can move to that scheme in the future when we feel mozjs is stable enough.

[jdm]

The scheme makes sense to me.