Remove metadata from mkdocs.yml, allow mkdocs to build again

[brendo]

Currently, mkdocs build results in an error, this change fixes that 👍

mkdocs build --clean
INFO    -  Cleaning site directory 
INFO    -  Building documentation to directory: /Users/brendanabbott/Sites/mkdocs-material/site 
ERROR   -  file not found: /Users/brendanabbott/Sites/mkdocs-material/docs/extensions/metadata.md 
ERROR   -  Error building page extensions/metadata.md 
Traceback (most recent call last):
  File "/usr/local/bin/mkdocs", line 9, in <module>
    load_entry_point('mkdocs', 'console_scripts', 'mkdocs')()
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 716, in __call__
    return self.main(*args, **kwargs)
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 696, in main
    rv = self.invoke(ctx)
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 1060, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 889, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/usr/local/lib/python2.7/site-packages/click/core.py", line 534, in invoke
    return callback(*args, **kwargs)
  File "/Users/brendanabbott/Sites/mkdocs/mkdocs/__main__.py", line 156, in build_command
    ), dirty=not clean)
  File "/Users/brendanabbott/Sites/mkdocs/mkdocs/commands/build.py", line 379, in build
    build_pages(config, dirty=dirty)
  File "/Users/brendanabbott/Sites/mkdocs/mkdocs/commands/build.py", line 332, in build_pages
    dump_json)
  File "/Users/brendanabbott/Sites/mkdocs/mkdocs/commands/build.py", line 179, in _build_page
    input_content = io.open(input_path, 'r', encoding='utf-8').read()
IOError: [Errno 2] No such file or directory: '/Users/brendanabbott/Sites/mkdocs-material/docs/extensions/metadata.md'

[brendo]

Sidenote, perhaps a test should be added to travis to attempt to build mkdocs to prevent this from happening in future?

edit I noticed that the mkdocs:build gulp task is actually called, but the error is not bubbled up. This could happen by using stdio (similar to the serve script) which would give visibility, but not stop the build. Alternatively we could peek at the result of spawnSync and if the status is not 0, throw an error which would cause gulp to exit with a non zero code, and have it fail 👍

[squidfunk]

Thanks! Indeed the build should have errored. I will look into it.

[squidfunk]

Build error bubbling fixed in f56b070