Run activate using call on appveyor

Author: isuruf

appveyor.yml

@@ -52,7 +52,7 @@ install:
 - if [%COMPILER%]==[MSVC15] conda config --add channels conda-forge
 - if [%COMPILER%]==[MSVC15] if [%BUILD_TYPE%]==[Debug] conda config --add channels symengine/label/debug
 - if [%COMPILER%]==[MSVC15] conda create -n test --yes mpir=3.0.0 vc=14
-- if [%COMPILER%]==[MSVC15] activate test
+- if [%COMPILER%]==[MSVC15] call activate test
 - if [%COMPILER%]==[MSVC15] if [%WITH_MPFR%]==[yes] conda install --yes mpfr=3.1.5
 - if [%COMPILER%]==[MSVC15] if [%WITH_MPC%]==[yes] conda install --yes mpc=1.0.3
 - if [%COMPILER%]==[MSVC15] if [%WITH_LLVM%]==[yes] conda install --yes llvmdev=3.9