Adds incremental iOS versioning option.

Setting `versioning_method` on `deployment_target` to *incremental* will use `agvtool`'s `next-version` option to bump build numbers in integer increments. Anything else (or blank) will use the current git SHA.

From `avgtool`'s docs:
>Increments the version numbers of all versioned targets to the next highest integral value.  For example, 54
>will change to 55 and 234.6 will change to 235.

Requires `CURRENT_PROJECT_VERSION` to be set in `<project-name>.xcodeproj`. 
Can be changed via *Project* -> *Build Settings* -> *Versioning* -> *Current Project Version*.