COMP: Do not pip install --upgrade virtualenv packages.

Python lacks a decent build system and can easily fail to build.
COMP: MetaClassPDF size_t to unsigned int.

For:

  ..\..\..\..\TubeTK\Base\IO\itktubeMetaClassPDF.cxx(779): warning C4267:
  'initializing' : conversion from 'size_t' to 'unsigned int', possible loss
  of data
COMP: SyncRecord size_t to int possible loss of data.
Merge pull request #152 from thewtex/Windows-warnings

Windows warnings