Fix sage build

Author: isuruf

bin/install_travis.sh

@@ -19,9 +19,11 @@ fi
 
 if [[ "${WITH_SAGE}" == "yes" ]]; then
     # This is split to avoid the 10 minute limit
-    conda install -q sagelib
-    export conda_pkgs="${conda_pkgs} sage";
+    conda install -q sagelib=8.1
+    conda clean --all
+    export conda_pkgs="${conda_pkgs} sage=8.1";
 fi
 
 conda install -q ${conda_pkgs}
+conda clean --all
 source activate $our_install_dir;