[WIP] numexpr failure

I am trying to add numexpr, but this is what I get from clean state:
```
$ ./update
[...]
[matplotlib] running ['hit', 'build-postprocess', '--shebang=multiline', '--write-protect']
Removing build dir: /auto/nest/nest/u/ondrej/repos/python-hpcmp2/bld/matplotlib-n-w43t
Downloading sources for numexpr
warning: no common commits
remote: Counting objects: 901, done.
remote: Compressing objects: 100% (682/682), done.
remote: Total 901 (delta 388), reused 704 (delta 212)
Receiving objects: 100% (901/901), 283.73 KiB | 534 KiB/s, done.
Resolving deltas: 100% (388/388), done.
From git://github.com/erdc-cm/numexpr
 * branch            master     -> FETCH_HEAD
[ERROR] Uncaught exception:
Traceback (most recent call last):
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/_hashdist/hashdist/cli/main.py", line 132, in help_on_exceptions
    return func(*args, **kw)
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/builder/builder.py", line 173, in main
    profile_aid = ctx.build_all(packages, 'profile')
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/builder/builder.py", line 58, in build_all
    dep_spec = visit(root_name)
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/builder/builder.py", line 49, in visit
    imports.append(visit(dep))
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/builder/builder.py", line 51, in visit
    artifact_id = build_package(self, pkg, imports)
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/builder/builder.py", line 105, in build_package
    download_sources(ctx, pkg)
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/builder/builder.py", line 67, in download_sources
    ctx.source_cache.fetch(pkg['url'], pkg['key'])
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/_hashdist/hashdist/core/source_cache.py", line 305, in fetch
    handler.fetch(url, type, hash)
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/_hashdist/hashdist/core/source_cache.py", line 439, in fetch
    self.fetch_git(repo, branch, commit)
  File "/auto/nest/nest/u/ondrej/repos/python-hpcmp2/_hashdist/hashdist/core/source_cache.py", line 470, in fetch_git
    (repository, commit))
SourceNotFoundError: Repository "git://github.com/erdc-cm/numexpr.git" did not contain commit "c452bb8fa8aac87b89d131f5d542cecea522a239"

This exception has not been translated to a human-friendly error message,
please file an issue at https://github.com/hashdist/hashdist/issues pasting
this stack trace.
```
But clearly the commit is there (see #16).

@dagss, would you mind trying this branch on your computer, to see if you get the same error?