Update `get-pip.py` URLs in `pyenv-virtualenv` (#426)

bin/pyenv-virtualenv

@@ -485,13 +485,22 @@ if [ -z "${GET_PIP_URL}" ]; then
       GET_PIP_URL="https://bootstrap.pypa.io/pip/2.7/get-pip.py"
       ;;
     3.2 | 3.2.* )
-      GET_PIP_URL="https://bootstrap.pypa.io/3.2/get-pip.py"
+      GET_PIP_URL="https://bootstrap.pypa.io/pip/3.2/get-pip.py"
       ;;
     3.3 | 3.3.* )
-      GET_PIP_URL="https://bootstrap.pypa.io/3.3/get-pip.py"
+      GET_PIP_URL="https://bootstrap.pypa.io/pip/3.3/get-pip.py"
+      ;;
+    3.4 | 3.4.* )
+      GET_PIP_URL="https://bootstrap.pypa.io/pip/3.4/get-pip.py"
+      ;;
+    3.5 | 3.5.* )
+      GET_PIP_URL="https://bootstrap.pypa.io/pip/3.5/get-pip.py"
+      ;;
+    3.6 | 3.6.* )
+      GET_PIP_URL="https://bootstrap.pypa.io/pip/3.6/get-pip.py"
       ;;
     * )
-      GET_PIP_URL="https://bootstrap.pypa.io/get-pip.py"
+      GET_PIP_URL="https://bootstrap.pypa.io/pip/get-pip.py"
       ;;
     esac
   fi