Fix Appveyor branch handling

appveyor.yml

@@ -32,14 +32,16 @@ environment:
 build: off
 
 install:
+  - set BOOST_BRANCH=develop
+  - if "%APPVEYOR_REPO_BRANCH%" == "master" set BOOST_BRANCH=master
   - cd ..
-  - git clone -b %APPVEYOR_REPO_BRANCH% https://github.com/boostorg/boost.git boost-root
+  - git clone -b %BOOST_BRANCH% --depth 1 https://github.com/boostorg/boost.git boost-root
   - cd boost-root
   - git submodule update --init tools/boostdep
   - xcopy /s /e /q %APPVEYOR_BUILD_FOLDER% libs\preprocessor\
   - python tools/boostdep/depinst/depinst.py preprocessor
   - bootstrap
-  - b2 headers
+  - b2 -d0 headers
 
 for:
 -