Require cmake>=3.1

Plus some other build-related changes. I don't think there is anything
functionally different from 1.7.7, or even any binary incompatibilities, but
the cmake change is significant.

1.7.6 <- 1.7.5

1.7.5 <- 1.7.4

1.7.4 <- 1.7.3

Merge pull request open-source-parsers#490 from open-source-parsers/n…

…ull-singleton

Use a Myers Singleton for null

0.10.6 <- 0.10.5

1.7.2 <- 1.7.1

This only fixes a clang warning, but we have already
seen more than one report for it.

1.7.1 <- 1.7.0

And 1.7.0 is recalled b/c we accidentally
had SecureAlloc by default.

Merge pull request open-source-parsers#442 from open-source-parsers/J…

…SONCPP_STRING

Secure allocator available for wiping memory.

Resolves open-source-parsers#437.
Resolves open-source-parsers#152.