Twenty Fifteen: Stop before 4.1 to activate the theme

[iamtakashi]

As a part of effort to make people update their install to the latest version, let's not allow before 4.1 to activate the theme.

[iamtakashi]

Limit back compat to 4.1 and later versions.

[iandstewart]

In 30099:

Twenty Fifteen: avoid errors by preventing Twenty Fifteen from being activated in old versions of WordPress

Props iamtakashi, fixes #30137.

[iandstewart]

This is breaking the customizer right now. I'll leave it in but comment it out in functions.php.

[iandstewart]

In 30103:

Twenty Fifteen: back compat checks in Twenty Fifteen are breaking the customizer, leaving them in for now, but commented out.

See #30137.

[SergeyBiryukov]

Looking at [23825], we probably just need to change 4.1 to 4.1-alpha.

[SergeyBiryukov]

In 30116:

Twenty Fifteen: Change version compare string to '4.1-alpha' to avoid an error for alpha and beta testers during development.

fixes #30137.