cannot build documentation with the latest nimble HEAD
Running `$ nimble gen_docs` with the latest nimble HEAD  https://github.com/nim-lang/nimble/commit/f4c818a046c2c3ceb1523c4c974e4b6e625d8ca3
results in the following error:
```
command: nimble gen_docs
  Executing task gen_docs in /Users/xxxx/dev/nim/pkg/arraymancer/arraymancer.nimble
stack trace: (most recent call last)
/private/var/folders/dt/5vm4d3sj5ll52x507rkfkw6r0000gn/T/nimblecache-2996282066/nimscriptapi_1468343325.nim(187, 16)
/Users/xxxx/dev/nim/pkg/arraymancer/arraymancer.nimble(248, 14) gen_docsTask
/Users/xxxx/dev/nim/pkg/arraymancer/docs/docs.nim(156, 20) buildDocs
/Users/xxxx/dev/nim/pkg/arraymancer/docs/docs.nim(44, 11) execAction
/Users/xxxx/.choosenim/toolchains/nim-1.2.6/lib/system/assertions.nim(29, 26) failedAssertImpl
/Users/xxxx/.choosenim/toolchains/nim-1.2.6/lib/system/assertions.nim(22, 11) raiseAssert
/Users/xxxx/.choosenim/toolchains/nim-1.2.6/lib/system/fatal.nim(49, 5) sysFatal
/Users/xxxx/.choosenim/toolchains/nim-1.2.6/lib/system/fatal.nim(49, 5) Error: unhandled exception: /Users/xxxx/dev/nim/pkg/arraymancer/docs/docs.nim(44, 12) `ret == 0` Command failed: 1
cmd: /Users/xxxx/.choosenim/toolchains/nim-1.2.6/bin/nim buildIndex -o:/private/var/folders/dt/5vm4d3sj5ll52x507rkfkw6r0000gn/T/nimblecache-2996282066/arraymancer_1183897761/docs/build/theindex.html /private/var/folders/dt/5vm4d3sj5ll52x507rkfkw6r0000gn/T/nimblecache-2996282066/arraymancer_1183897761/docs/build
result:
oserr.nim(94)            raiseOSError
Error: unhandled exception: No such file or directory
Additional info: "/private/var/folders/dt/5vm4d3sj5ll52x507rkfkw6r0000gn/T/nimblecache-2996282066/arraymancer_1183897761/docs" [OSError] [AssertionError]
     Error: Exception raised during nimble script execution
```
No documentation is built.
--Leo