[MRG] Always download latest Miniconda on Travis (scikit-learn#8857)

Author: naoyak

build_tools/travis/install.sh

@@ -37,12 +37,9 @@ if [[ "$DISTRIB" == "conda" ]]; then
     echo "Cached in $HOME/download :"
     ls -l
     echo
-    if [[ ! -f miniconda.sh ]]
-        then
-        wget http://repo.continuum.io/miniconda/Miniconda-3.6.0-Linux-x86_64.sh \
-            -O miniconda.sh
-        fi
-    chmod +x miniconda.sh && ./miniconda.sh -b
+    wget https://repo.continuum.io/miniconda/Miniconda3-latest-Linux-x86_64.sh \
+        -O miniconda.sh
+    chmod +x miniconda.sh && ./miniconda.sh -b -p /home/travis/miniconda
     cd ..
     export PATH=/home/travis/miniconda/bin:$PATH
     conda update --yes conda