Fix typo in appveyor.yml

Author: whtsky

appveyor.yml

@@ -42,7 +42,7 @@ install:
   - "SET PATH=%PYTHON%;%PYTHON%\\scripts;%PATH%"
   - echo "C:\Program Files\Microsoft SDKs\Windows\v7.1\Bin\SetEnv.cmd" /x64 > "C:\Program Files (x86)\Microsoft Visual Studio 10.0\VC\bin\amd64\vcvars64.bat"
   - "pip install --disable-pip-version-check --user --upgrade pip wheel"
-  - "pip install --user -r requirements.txt"
+  - "pip install --user -r dev-requirements.txt"
 
 build_script:
   - "cython bencoder.pyx"