Update pip.

appveyor.yml

@@ -31,6 +31,7 @@ install:
   - 'conda install --yes --quiet numpy scipy matplotlib setuptools wheel'
   - 'conda install --yes --quiet --channel matsci spglib netcdf4'
   - 'conda update --yes --all'
+  - 'pip install pip --upgrade'
   - "pip install -r requirements.txt"
   - "pip install pybtex nose coverage ase coveralls fdint ase twine networkx"  # Note that pyhull is excluded because it does not work on Windows.
   - "pip install pybtex nose coverage ase coveralls fdint ase phonopy twine networkx"  # Note that pyhull is excluded because it does not work on Windows.