is_convertible_v -> is_convertible::value

[yakra]

This allows compilation using older toolchains. For example, I'm using clang++ 9.0.1 on a couple machines with header files from GCC 5 & 4.8.2, which don't define is_convertible_v.

[vitaut]

Thanks for the PR. In general mixing newer compiler with older library standard is not recommended but considering that this is a simple workaround, I'm OK with merging this.

[yakra]

Thanks for the merge!