Bump minimum version

[InfectedBytes]

Supersede loicteixeira#83

So far, the minimum version to use the API is Unity 5.0.1 (as of v2.1.0) which means the package has to be generated with Unity 5.0.1.

It's probably not necessary to support that far back and could probably be bumped to something more recent. Unity 2017 maybe?

[InfectedBytes]

Bumping the minimum version is a good idea, but I'm not sure if we should jump directly to Unity 2017.
Unity 2017 is awesome, but I think there are still many users in the Unity 5 cycle.
I don't have fresh numbers of the usage distribution, but in June it kinda looked like this:
Unity 2017: 14%
Unity 5.6: 51%
Unity 5.5: 18%
Unity 5.4: 8%
Unity 5.3: 5%
Unity 5.2: 1%
<Unity 5.2: ~3%
So we could savely ignore all versions below 5.3 and still cover 95%.
We could even go up to 5.5 (covering 83%), but I think we should not bump the minimum version above that (at least for now)

[InfectedBytes]

I can't really believe it, but as it seems the version usage has shifted drastically:
2017.3: ~47%
2017.2: ~9%
2017.1: ~7%
5.6: 20%
5.5: ~5%
5.4: ~3%
5.3: ~4%
others: ~5%

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As of June 15:
2018.x: ~29%
2017.x: ~48%
5.6: ~14%
5.5: ~3%
5.4: ~2%
5.3: ~2%
others: ~2%

[InfectedBytes]

Minimum Version is now 5.5.
Sorry to those 6% that are using an older version 😕