travis: use py 3.5.3 and 3.6.7 to avoid openssl 1.1 issues.

it seems that pyenv now depends on openssl 1.1, so use python versions
that work with openssl 1.1 for testing (same versions as currently
in master branch).

.travis/install.sh

@@ -30,12 +30,12 @@ if [[ "$(uname -s)" == 'Darwin' ]]; then
             pyenv global 3.4.5
             ;;
         py35)
-            pyenv install 3.5.2
-            pyenv global 3.5.2
+            pyenv install 3.5.3  # minimum for openssl 1.1.x
+            pyenv global 3.5.3
             ;;
         py36)
-            pyenv install 3.6.0
-            pyenv global 3.6.0
+            pyenv install 3.6.7  # minimum for homebrew to select openssl 1.1.x
+            pyenv global 3.6.7
             ;;
     esac
     pyenv rehash